Network Working Group S. Legg
Request for Comments: 4910 eB2Bcom
Category: Experimental D. Prager
July 2007
Robust XML Encoding Rules (RXER) for
Abstract Syntax Notation One (ASN.1)
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document defines a set of Abstract Syntax Notation One (ASN.1)
encoding rules, called the Robust XML Encoding Rules or RXER, that
produce an Extensible Markup Language (XML) representation for values
of any given ASN.1 data type. Rules for producing a canonical RXER
encoding are also defined.
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Table of Contents
1. Introduction ....................................................3
2. Conventions .....................................................4
3. Definitions .....................................................5
4. Additional Basic Types ..........................................6
4.1. The Markup Type ............................................6
4.1.1. Self-Containment ....................................9
4.1.2. Normalization for Canonical Encoding Rules .........12
4.2. The AnyURI Type ...........................................13
4.3. The NCName Type ...........................................14
4.4. The Name Type .............................................14
4.5. The QName Type ............................................14
5. Expanded Names for ASN.1 Types .................................15
6. Encoding Rules .................................................17
6.1. Identifiers ...............................................19
6.2. Component Encodings .......................................20
6.2.1. Referenced Components ..............................20
6.2.2. Element Components .................................20
6.2.2.1. Namespace Properties for Elements .........22
6.2.2.2. Namespace Prefixes for Element Names ......24
6.2.3. Attribute Components ...............................25
6.2.3.1. Namespace Prefixes for Attribute Names ....26
6.2.4. Unencapsulated Components ..........................26
6.2.5. Examples ...........................................27
6.3. Standalone Encodings ......................................28
6.4. Embedded ASN.1 Values .....................................28
6.5. Type Referencing Notations ................................32
6.6. TypeWithConstraint, SEQUENCE OF Type, and SET OF Type .....33
6.7. Character Data Translations ...............................34
6.7.1. Restricted Character String Types ..................35
6.7.2. BIT STRING .........................................36
6.7.3. BOOLEAN ............................................38
6.7.4. ENUMERATED .........................................38
6.7.5. GeneralizedTime ....................................39
6.7.6. INTEGER ............................................41
6.7.7. NULL ...............................................42
6.7.8. ObjectDescriptor ...................................43
6.7.9. OBJECT IDENTIFIER and RELATIVE-OID .................43
6.7.10. OCTET STRING ......................................43
6.7.11. QName .............................................44
6.7.11.1. Namespace Prefixes for Qualified Names ...44
6.7.12. REAL ..............................................45
6.7.13. UTCTime ...........................................46
6.7.14. CHOICE as UNION ...................................47
6.7.15. SEQUENCE OF as LIST ...............................50
6.8. Combining Types ...........................................50
6.8.1. CHARACTER STRING ...................................51
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6.8.2. CHOICE .............................................51
6.8.3. EMBEDDED PDV .......................................52
6.8.4. EXTERNAL ...........................................52
6.8.5. INSTANCE OF ........................................52
6.8.6. SEQUENCE and SET ...................................52
6.8.7. SEQUENCE OF and SET OF .............................54
6.8.8. Extensible Combining Types .........................55
6.8.8.1. Unknown Elements in Extensions ............55
6.8.8.2. Unknown Attributes in Extensions ..........59
6.9. Open Type .................................................60
6.10. Markup ...................................................61
6.11. Namespace Prefixes for CRXER .............................63
6.12. Serialization ............................................65
6.12.1. Non-Canonical Serialization .......................65
6.12.2. Canonical Serialization ...........................68
6.12.3. Unicode Normalization in XML Version 1.1 ..........70
6.13. Syntax-Based Canonicalization ............................70
7. Transfer Syntax Identifiers ....................................71
7.1. RXER Transfer Syntax ......................................71
7.2. CRXER Transfer Syntax .....................................71
8. Relationship to XER ............................................71
9. Security Considerations ........................................73
10. Acknowledgements ..............................................74
11. IANA Considerations ...........................................75
12. References ....................................................75
12.1. Normative References .....................................75
12.2. Informative References ...................................77
Appendix A. Additional Basic Definitions Module ...................78
1. Introduction
This document defines a set of Abstract Syntax Notation One (ASN.1)
[X.680] encoding rules, called the Robust XML Encoding Rules or RXER,
that produce an Extensible Markup Language (XML) [XML10][XML11]
representation of ASN.1 values of any given ASN.1 type.
An ASN.1 value is regarded as analogous to the content and attributes
of an XML element, or in some cases, just an XML attribute value.
The RXER encoding of an ASN.1 value is the well-formed and valid
content and attributes of an element, or an attribute value, in an
XML document [XML10][XML11] conforming to XML namespaces
[XMLNS10][XMLNS11]. Simple ASN.1 data types such as PrintableString,
INTEGER, and BOOLEAN define character data content or attribute
values, while the ASN.1 combining types (i.e., SET, SEQUENCE, SET OF,
SEQUENCE OF, and CHOICE) define element content and attributes. The
attribute and child element names are generally provided by the
identifiers of the components in combining type definitions, i.e.,
elements and attributes correspond to the NamedType notation.
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RXER leaves some formatting details to the discretion of the encoder,
so there is not a single unique RXER encoding for an ASN.1 value.
However, this document also defines a restriction of RXER, called the
Canonical Robust XML Encoding Rules (CRXER), which does produce a
single unique encoding for an ASN.1 value. Obviously, the CRXER
encoding of a value is also a valid RXER encoding of that value. The
restrictions on RXER to produce the CRXER encoding are interspersed
with the description of the rules for RXER.
Note that "ASN.1 value" does not mean a Basic Encoding Rules (BER)
[X.690] encoding. The ASN.1 value is an abstract concept that is
independent of any particular encoding. BER is just one possible way
to encode an ASN.1 value. This document defines an alternative way
to encode an ASN.1 value.
A separate document [RXEREI] defines encoding instructions [X.680-1]
that may be used in an ASN.1 specification to modify how values are
encoded in RXER, for example, to encode a component of a combining
ASN.1 type as an attribute rather than as a child element. A
pre-existing ASN.1 specification will not have RXER encoding
instructions, so any mention of encoding instructions in this
document can be ignored when dealing with such specifications.
Encoding instructions for other encoding rules have no effect on RXER
encodings.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", and "MAY" in this document are
to be interpreted as described in BCP 14, RFC 2119 [BCP14]. The key
word "OPTIONAL" is exclusively used with its ASN.1 meaning.
A reference to an ASN.1 production [X.680] (e.g., Type, NamedType) is
a reference to the text in an ASN.1 specification corresponding to
that production.
The specification of RXER makes use of definitions from the XML
Information Set (Infoset) [INFOSET]. In particular, information item
property names follow the Infoset convention of being shown in square
brackets, e.g., [local name]. Literal values of Infoset properties
are enclosed in double quotes; however, the double quotes are not
part of the property values. In the sections that follow,
"information item" will be abbreviated to "item", e.g., "element
information item" is abbreviated to "element item". The term
"element" or "attribute" (without the "item") is referring to an
element or attribute in an XML document, rather than an information
item.
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Literal character strings to be used in an RXER encoding appear
within double quotes; however, the double quotes are not part of the
literal value and do not appear in the encoding.
This document uses the namespace prefix [XMLNS10][XMLNS11] "asnx:" to
stand for the namespace name "urn:ietf:params:xml:ns:asnx", uses the
namespace prefix "xs:" to stand for the namespace name
"http://www.w3.org/2001/XMLSchema", and uses the namespace prefix
"xsi:" to stand for the namespace name
"http://www.w3.org/2001/XMLSchema-instance". However, in practice,
any valid namespace prefixes are permitted in non-canonical RXER
encodings (namespace prefixes are deterministically generated for
CRXER).
The encoding instructions [X.680-1] referenced by name in this
specification are encoding instructions for RXER [RXEREI].
Throughout this document, references to the Markup, AnyURI, NCName,
Name, and QName ASN.1 types are references to the types described in
Section 4 and consolidated in the AdditionalBasicDefinitions module
in Appendix A. Any provisions associated with the reference do not
apply to types defined in other ASN.1 modules that happen to have
these same names.
Code points for characters [UCS][UNICODE] are expressed using the
Unicode convention U+n, where n is four to six hexadecimal digits,
e.g., the space character is U+0020.
3. Definitions
Definition (white space character): A white space character is a
space (U+0020), tab (U+0009), carriage return (U+000D), or line feed
(U+000A) character.
Definition (white space): White space is a sequence of one or more
white space characters.
Definition (line break): A line break is any sequence of characters
that is normalized to a line feed by XML End-of-Line Handling
[XML10][XML11].
Definition (serialized white space): Serialized white space is a
sequence of one or more white space characters and/or line breaks.
Definition (declaring the default namespace): A namespace
declaration attribute item is declaring the default namespace if the
[prefix] of the attribute item has no value, the [local name] of the
attribute item is "xmlns" and the [normalized value] is not empty.
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Definition (undeclaring the default namespace): A namespace
declaration attribute item is undeclaring the default namespace if
the [prefix] of the attribute item has no value, the [local name] of
the attribute item is "xmlns" and the [normalized value] is empty
(i.e., xmlns="").
Definition (canonical namespace prefix): A canonical namespace prefix
is an NCName [XMLNS10] beginning with the letter 'n' (U+006E)
followed by a non-negative number string. A non-negative number
string is either the digit character '0' (U+0030), or a non-zero
decimal digit character (U+0031-U+0039) followed by zero, one, or
more of the decimal digit characters '0' to '9' (U+0030-U+0039).
For convenience, a CHOICE type where the ChoiceType is subject to a
UNION encoding instruction will be referred to as a UNION type, and a
SEQUENCE OF type where the SequenceOfType is subject to a LIST
encoding instruction will be referred to as a LIST type.
4. Additional Basic Types
This section defines an ASN.1 type for representing markup in
abstract values, as well as basic types that are useful in encoding
instructions [RXEREI] and other related specifications [ASN.X].
The ASN.1 definitions in this section are consolidated in the
AdditionalBasicDefinitions ASN.1 module in Appendix A.
4.1. The Markup Type
A value of the Markup ASN.1 type holds the [prefix], [attributes],
[namespace attributes], and [children] of an element item, i.e., the
content and attributes of an element.
RXER has special provisions for encoding values of the Markup type
(see Section 6.10). For other encoding rules, a value of the Markup
type is encoded according to the following ASN.1 type definition
(with AUTOMATIC TAGS):
Markup ::= CHOICE {
text SEQUENCE {
prolog UTF8String (SIZE(1..MAX)) OPTIONAL,
prefix NCName OPTIONAL,
attributes UTF8String (SIZE(1..MAX)) OPTIONAL,
content UTF8String (SIZE(1..MAX)) OPTIONAL
}
}
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The text alternative of the Markup CHOICE type provides for the
[prefix], [attributes], [namespace attributes], and [children] of an
element item to be represented as serialized XML using the UTF-8
character encoding [UTF-8].
Aside: The CHOICE allows for one or more alternative compact
representations of the content and attributes of an element to be
supported in a future specification.
With respect to some element item whose content and attributes are
represented by a value of the text alternative of the Markup type:
(1) the prolog component of the value contains text that, after line
break normalization, conforms to the XML prolog production
[XML10][XML11],
(2) the prefix component is absent if the [prefix] of the element
item has no value; otherwise, the prefix component contains the
[prefix] of the element item,
(3) the attributes component of the value contains an XML
serialization of the [attributes] and [namespace attributes] of
the element item, if any, with each attribute separated from the
next by serialized white space, and
(4) the content component is absent if the [children] property of the
element item is empty; otherwise, the content component of the
value contains an XML serialization of the [children] of the
element item.
All the components of a value of the Markup type MUST use the same
version of XML, either version 1.0 [XML10] or version 1.1 [XML11].
If XML version 1.1 is used, then the prolog component MUST be present
and MUST have an XMLDecl for version 1.1. If the prolog component is
absent, then XML version 1.0 is assumed.
If the prefix component is present, then there MUST be a namespace
declaration attribute in the attributes component that defines that
namespace prefix (since an element whose content and attributes are
described by a value of Markup is required to be self-contained; see
Section 4.1.1).
Note that the prefix component is critically related to the NamedType
that has Markup as its type. If a Markup value is extracted from one
enclosing abstract value and embedded in another enclosing abstract
value (i.e., becomes associated with a different NamedType), then the
prefix may no longer be appropriate, in which case it will need to be
revised. It may also be necessary to add another namespace
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declaration attribute to the attributes component so as to declare a
new namespace prefix.
Leading and/or trailing serialized white space is permitted in the
attributes component. A value of the attributes component consisting
only of serialized white space (i.e., no actual attributes) is
permitted.
The attributes and content components MAY contain entity references
[XML10][XML11]. If any entity references are used (other than
references to the predefined entities), then the prolog component
MUST be present and MUST contain entity declarations for those
entities in the internal or external subset of the document type
definition.
Example
Given the following ASN.1 module:
MyModule DEFINITIONS
AUTOMATIC TAGS ::= BEGIN
Message ::= SEQUENCE {
messageType INTEGER,
messageValue Markup
}
ENCODING-CONTROL RXER
TARGET-NAMESPACE "http://example.com/ns/MyModule"
COMPONENT message Message
-- a top-level NamedType
END
consider the following XML document:
]>
1&TRUE;
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A Markup value corresponding to the content and attributes of the
element is, in ASN.1 value notation [X.680] (where
"lf" represents the line feed character):
text:{
prolog { "", lf,
"", lf,
" ", lf,
"]>", lf },
attributes { " xmlns:ns=""http://www.example.com/ABD""",
lf,
" ns:foo=""1"" bar=""0""" },
content { lf,
" &TRUE;", lf,
" ", lf, " " }
}
The following Markup value is an equivalent representation of the
content and attributes of the element:
text:{
attributes {
"bar=""0"" ns:foo=""1"" ",
"xmlns:ns=""http://www.example.com/ABD""" },
content { lf,
" true", lf,
" ", lf, " " }
}
By itself, the Markup ASN.1 type imposes no data type restriction on
the markup contained by its values and is therefore analogous to the
XML Schema anyType [XSD1].
There is no ASN.1 basic notation that can directly impose the
constraint that the markup represented by a value of the Markup type
must conform to the markup allowed by a specific type definition.
However, certain encoding instructions (i.e., the reference encoding
instructions [RXEREI]) have been defined to have this effect.
4.1.1. Self-Containment
An element, its attributes and its content, including descendent
elements, may contain qualified names [XMLNS10][XMLNS11] as the names
of elements and attributes, in the values of attributes, and as
character data content of elements. The binding between namespace
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prefix and namespace name for these qualified names is potentially
determined by the namespace declaration attributes of ancestor
elements (which in the Infoset representation are inherited as
namespace items in the [in-scope namespaces]).
In the absence of complete knowledge of the data type of an element
item whose content and attributes are described by a value of the
Markup type, it is not possible to determine with absolute certainty
which of the namespace items inherited from the [in-scope namespaces]
of the [parent] element item are significant in interpreting the
Markup value. The safe and easy option would be to assume that all
the namespace items from the [in-scope namespaces] of the [parent]
element item are significant and need to be retained within the
Markup value. When the Markup value is re-encoded, any of the
retained namespace items that do not appear in the
[in-scope namespaces] of the enclosing element item in the new
encoding could be made to appear by outputting corresponding
namespace declaration attribute items in the [namespace attributes]
of the enclosing element item.
From the perspective of the receiver of the new encoding, this
enlarges the set of attribute items in the [namespace attributes]
represented by the Markup value.
In addition, there is no guarantee that the sender of the new
encoding has recreated the original namespace declaration attributes
on the ancestor elements, so the [in-scope namespaces] of the
enclosing element item is likely to have new namespace declarations
that the receiver will retain and pass on in the
[namespace attributes] when it in turn re-encodes the Markup value.
This unbounded growth in the set of attribute items in the
[namespace attributes] defeats any attempt to produce a canonical
encoding.
The principle of self-containment is introduced to avoid this
problem. An element item (the subject element item) is
self-contained if the constraints of Namespaces in XML 1.0 [XMLNS10]
are satisfied (i.e., that prefixes are properly declared) and none of
the following bindings are determined by a namespace declaration
attribute item in the [namespace attributes] of an ancestor element
item of the subject element item:
(1) the binding between the [prefix] and [namespace name] of the
subject element item,
(2) the binding between the [prefix] and [namespace name] of any
descendant element item of the subject element item,
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(3) the binding between the [prefix] and [namespace name] of any
attribute item in the [attributes] of the subject element item or
the [attributes] of any descendant element item of the subject
element item,
(4) the binding between the namespace prefix and namespace name of
any qualified name in the [normalized value] of any attribute
item in the [attributes] of the subject element item or the
[attributes] of any descendant element item of the subject
element item, or
(5) the binding between the namespace prefix and namespace name of
any qualified name represented by a series of character items
(ignoring processing instruction and comment items) in the
[children] of the subject element item or the [children] of any
descendant element item of the subject element item.
Aside: If an element is self-contained, then separating the
element from its parent does not change the semantic
interpretation of its name and any names in its content and
attributes.
A supposedly self-contained element in a received RXER encoding that
is in fact not self-contained SHALL be treated as an ASN.1 constraint
violation.
Aside: ASN.1 does not require an encoding with a constraint
violation to be immediately rejected; however, the constraint
violation must be reported at some point, possibly in a separate
validation step.
Implementors should note that an RXER decoder will be able to detect
some, but not all, violations of self-containment. For example, it
can detect element and attribute names that depend on namespace
declarations appearing in the ancestors of a supposedly
self-contained element. Similarly, where type information is
available, it can detect qualified names in character data that
depend on the namespace declarations of ancestor elements. However,
type information is not always available, so some qualified names
will escape constraint checking. Thus, the onus is on the creator of
the original encoding to ensure that element items required to be
self-contained really are completely self-contained.
An element item whose content and attributes are described by a value
of the Markup type MUST be self-contained.
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Aside: The procedures in Section 6 take account of the
requirements for self-containment so that an RXER encoder
following these procedures will not create violations of
self-containment.
4.1.2. Normalization for Canonical Encoding Rules
Implementations are given some latitude in how the content and
attributes of an element are represented as an abstract value of the
Markup type, in part because an Infoset can have different equivalent
serializations. For example, the order of attributes and the amount
and kind of white space characters between attributes are irrelevant
to the Infoset representation. The content can also include one or
more elements corresponding to an ASN.1 top-level NamedType or having
a data type that is an ASN.1 type. It is only necessary to preserve
the abstract value for such elements, and a particular abstract value
can have different Infoset representations.
These two characteristics mean that when an RXER encoded value of the
Markup type is decoded, the components of the recovered Markup value
may not be exactly the same, character for character, as the original
value that was encoded, though the recovered value will be
semantically equivalent.
However, canonical ASN.1 encoding rules such as the Distinguished
Encoding Rules (DER) and the Canonical Encoding Rules (CER) [X.690],
which encode Markup values according to the ASN.1 definition of the
Markup type, depend on character-for-character preservation of string
values. This requirement can be accommodated if values of the Markup
type are normalized when they are encoded according to a set of
canonical encoding rules.
Aside: The RXER encoding and decoding of a Markup value might
change the character string components of the value from the
perspective of BER, but there will be a single, repeatable
encoding for DER.
A value of the Markup type will appear as the content and attributes
of an element in an RXER encoding. When the value is encoded using a
set of ASN.1 canonical encoding rules other than CRXER, the
components of the text alternative of the value MUST be normalized as
follows, by reference to the element as it would appear in a CRXER
encoding:
(1) The value of the prolog component SHALL be the XMLDecl
with no other leading or trailing
characters.
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(2) If the element's name is unprefixed in the CRXER encoding, then
the prefix component SHALL be absent; otherwise, the value of the
prefix component SHALL be the prefix of the element's name in the
CRXER encoding.
(3) Take the character string representing the element's attributes,
including namespace declarations, in the CRXER encoding. If the
first attribute is a namespace declaration that undeclares the
default namespace (i.e., xmlns=""), then remove it. Remove any
leading space characters. If the resulting character string is
empty, then the attributes component SHALL be absent; otherwise,
the value of the attributes component SHALL be the resulting
character string.
Aside: Note that the attributes of an element can change if an
RXER encoding is re-encoded in CRXER.
(4) If the element has no characters between the start-tag and
end-tag [XML11] in the CRXER encoding, then the content component
SHALL be absent; otherwise, the value of the content component
SHALL be identical to the character string in the CRXER encoding
bounded by the element's start-tag and end-tag.
Aside: A consequence of invoking the CRXER encoding is that any
nested element corresponding to an ASN.1 top-level NamedType, or
indeed the element itself, will be normalized according to its
ASN.1 value rather than its Infoset representation. Likewise for
an element whose data type is an ASN.1 type. Section 6.4
describes how these situations can arise.
Aside: It is only through values of the Markup type that
processing instructions and comments can appear in CRXER
encodings.
If an application uses DER, but has no knowledge of RXER, then it
will not know to normalize values of the Markup type. If RXER is
deployed into an environment containing such applications, then
Markup values SHOULD be normalized, even when encoding using
non-canonical encoding rules.
4.2. The AnyURI Type
A value of the AnyURI ASN.1 type is a character string conforming to
the format of a Uniform Resource Identifier (URI) [URI].
AnyURI ::= UTF8String (CONSTRAINED BY
{ -- conforms to the format of a URI -- })
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4.3. The NCName Type
A value of the NCName ASN.1 type is a character string conforming to
the NCName production of Namespaces in XML 1.0 [XMLNS10].
NCName ::= UTF8String (CONSTRAINED BY
{ -- conforms to the NCName production of
-- Namespaces in XML 1.0 -- })
Aside: The NCName production for Namespaces in XML 1.1 [XMLNS11]
allows a wider range of characters than the NCName production for
Namespaces in XML 1.0. The NCName type for ASN.1 is currently
restricted to the characters allowed by Namespaces in XML 1.0,
though this may change in a future specification of RXER.
4.4. The Name Type
A value of the Name ASN.1 type is a character string conforming to
the Name production of XML version 1.0 [XML10].
Name ::= UTF8String (CONSTRAINED BY
{ -- conforms to the Name production of XML -- })
4.5. The QName Type
A value of the QName ASN.1 type describes an expanded name [XMLNS10],
which appears as a qualified name [XMLNS10] in an RXER encoding.
RXER has special provisions for encoding values of the QName type
(see Section 6.7.11). For other encoding rules, a value of the Qname
type is encoded according to the following ASN.1 type definition
(with AUTOMATIC TAGS):
QName ::= SEQUENCE {
namespace-name AnyURI OPTIONAL,
local-name NCName
}
The namespace-name component holds the namespace name of the expanded
name. If the namespace name of the expanded name has no value, then
the namespace-name component is absent.
Aside: A namespace name can be associated with ASN.1 types and
top-level NamedType instances by using the TARGET-NAMESPACE
encoding instruction.
The local-name component holds the local name of the expanded name.
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5. Expanded Names for ASN.1 Types
A TypeAssignment in ASN.1 associates a typereference with a Type.
For RXER and Abstract Syntax Notation X (ASN.X) [ASN.X], a
TypeAssignment is also regarded as associating an expanded name
[XMLNS10] with the Type. The local name of the expanded name is the
typereference on the left-hand side of the TypeAssignment. If the
target namespace [RXEREI] of the ASN.1 module in which the
TypeAssignment is defined is not absent, then the namespace name of
the expanded name is that target namespace; otherwise, the namespace
name of the expanded name has no value.
A Type that is a BuiltinType or ReferencedType that is one of the
productions in Table 1 is regarded as a reference to a built-in ASN.1
type. These built-in types also have expanded names. In each case,
the local name of the expanded name is as indicated in Table 1, and
the namespace name of the expanded name is
"urn:ietf:params:xml:ns:asnx".
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Table 1: Local Names for Built-in Types
+------------------------------------+-------------------+
| ASN.1 Production | Local Name |
+====================================+===================+
| BitStringType | |
| without a NamedBitList | BIT-STRING |
+------------------------------------+-------------------+
| BooleanType | BOOLEAN |
+------------------------------------+-------------------+
| CharacterStringType | |
| RestrictedCharacterStringType | |
| BMPString | BMPString |
| GeneralString | GeneralString |
| GraphicString | GraphicString |
| IA5String | IA5String |
| ISO646String | ISO646String |
| NumericString | NumericString |
| PrintableString | PrintableString |
| TeletexString | TeletexString |
| T61String | T61String |
| UniversalString | UniversalString |
| UTF8String | UTF8String |
| VideotexString | VideotexString |
| VisibleString | VisibleString |
| UnrestrictedCharacterStringType | CHARACTER-STRING |
+------------------------------------+-------------------+
| EmbeddedPDVType | EMBEDDED-PDV |
| ExternalType | EXTERNAL |
+------------------------------------+-------------------+
| IntegerType | |
| without a NamedNumberList | INTEGER |
+------------------------------------+-------------------+
| NullType | NULL |
| ObjectIdentifierType | OBJECT-IDENTIFIER |
| OctetStringType | OCTET-STRING |
| RealType | REAL |
| RelativeOIDType | RELATIVE-OID |
+------------------------------------+-------------------+
| UsefulType | |
| GeneralizedTime | GeneralizedTime |
| UTCTime | UTCTime |
| ObjectDescriptor | ObjectDescriptor |
+------------------------------------+-------------------+
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When the expanded name for an ASN.1 type is used in an RXER encoding,
it appears as a qualified name [XMLNS10][XMLNS11]. The namespace
prefix for the qualified name is determined according to
Section 6.7.11.1.
If a compatible XML Schema translation of an ASN.1 specification is
provided (see Section 6.4), then that schema SHOULD associate the
same expanded name with the XML Schema translation of an ASN.1 type.
Definition (namespace-qualified reference): An ASN.1 Type is a
namespace-qualified reference if one of the following applies:
(1) the Type is a typereference (not a DummyReference) or an
ExternalTypeReference in a DefinedType in a ReferencedType, the
ASN.1 module in which the referenced type is defined has a
TARGET-NAMESPACE encoding instruction, the referenced type is not
directly or indirectly an open type [X.681], and the referenced
type is not directly or indirectly the Markup type (Section 4.1),
or
(2) the Type is a BuiltinType or ReferencedType that is one of the
productions in Table 1.
The type definition referenced by a namespace-qualified reference
will have an expanded name with a value for the namespace name.
6. Encoding Rules
With respect to RXER, ASN.1 abstract values are uniformly regarded as
analogous to the content and attributes of an element, or just an
attribute value, not complete elements or attributes in their own
right. Elements and attributes in an RXER encoding are defined by
ASN.1 NamedType notation. Since elements are the fundamental
discrete structures of an XML document, the notion of a NamedType
having a value that can be encoded is useful for descriptive purposes
(particularly for describing the RXER encoding of values of the ASN.1
combining types). There is no conceptual basis in X.680 [X.680] for
talking about the value of a NamedType, or its encoding, so the
terminology is introduced here.
Definition (value of a NamedType): An abstract value of the Type in
a NamedType is also a value of that NamedType. The RXER encoding of
the value of a NamedType is the RXER encoding of the abstract value
of the Type encapsulated according to the definition of that
NamedType.
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This document does not refer to a value of a NamedType as being an
abstract value so as to remain consistent with X.680. An abstract
value is exclusively a value of an ASN.1 type.
A complete ASN.1 encoding is traditionally the encoding of an
abstract value, but it is more natural to think of an XML document as
being the RXER encoding of a value of a NamedType (because an XML
document has a single root element that contains all the other
elements and attributes). The ASN.1 basic notation does not allow a
NamedType to appear on its own, outside of an enclosing combining
type. That is, the basic notation does not have a concept analogous
to a global element or attribute definition. However, an ASN.1
specification may use an RXER encoding control section [RXEREI] to
define global elements and attributes using the NamedType notation.
A NamedType that is not contained in an ASN.1 type definition is
called a top-level NamedType [RXEREI]. Thus, an RXER encoding would
typically be described as the encoding of a value of a top-level
NamedType.
Section 6.2 describes how a value of a NamedType is encoded.
Section 6.3 defines an alternative method for encoding the document
element of an XML document when a top-level NamedType is not
specified. Section 6.4 describes how the encodings of ASN.1 values
can be embedded in an XML document where the other parts of the
document are validated by an XML Schema.
The RXER encoding of an abstract value, or the encoding of a value of
a NamedType, is described as a translation into a synthetic Infoset,
which is then serialized as XML. This separation has been chosen for
descriptive convenience and is not intended to impose any particular
architecture on RXER implementations. An RXER encoder is free to
encode an ASN.1 value directly to XML provided the result is
equivalent to following the two stage procedure described in this
document.
The process of translating an abstract value into an Infoset is
described as producing either:
(1) a string of characters that either becomes part of the
[normalized value] of an attribute item or becomes character
items among the [children] of an enclosing element item, or
(2) a collection of zero or more attribute items contributing to the
[attributes] of an enclosing element item, plus a series of zero
or more character, element, processing instruction (PI), or
comment items contributing to the [children] of the enclosing
element item.
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NamedType notation in the ASN.1 specification controls whether the
translation of an abstract value is encapsulated in an element item
or in an attribute item.
Sections 6.5 to 6.10 describe the translation of abstract values into
an Infoset for each of the ASN.1 type notations.
Section 6.11 describes post-processing of namespace prefixes for
CRXER encodings.
Section 6.12 specifies how the Infoset translation is serialized as
XML.
This specification assumes that the COMPONENTS OF transformation
specified in X.680, Clause 24.4 [X.680] has already been applied to
all relevant types.
Examples of RXER encodings in the following sections use a
start-tag and end-tag to hold attributes and delimit the
content. These start-tags and end-tags are for illustration only and
are not part of the encoding of an abstract value. In normal use,
the name of the enclosing element is provided by the context of the
type of the abstract value, e.g., a NamedType in an enclosing
SEQUENCE type.
An RXER decoder is a conforming XML processor [XML10][XML11].
6.1. Identifiers
An identifier, as defined in ASN.1 notation (Clause 11.3 of X.680
[X.680]), is a character string that begins with a Latin lowercase
letter (U+0061-U+007A) and is followed by zero, one or more Latin
letters (U+0041-U+005A, U+0061-U+007A), decimal digits (U+0030-
U+0039), and hyphens (U+002D). A hyphen is not permitted to be the
last character, and a hyphen is not permitted to be followed by
another hyphen. The case of letters in an identifier is always
significant.
ASN.1 identifiers are used for the [local name] of attribute and
element items, and may also appear in the character data content of
elements or the values of attributes. RXER encoding instructions can
be used to substitute an NCName [XMLNS10] for an identifier.
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6.2. Component Encodings
The translation of the value of a NamedType is the translation of the
abstract value of the Type of the NamedType encapsulated according to
the definition of that NamedType. This section specifies the form of
this encapsulation.
6.2.1. Referenced Components
A value of a NamedType that is subject to a COMPONENT-REF encoding
instruction is translated as a value of the top-level NamedType
referenced by the encoding instruction.
6.2.2. Element Components
A value of a NamedType that is not subject to an ATTRIBUTE,
ATTRIBUTE-REF, GROUP, or SIMPLE-CONTENT encoding instruction is
translated as an element item, either as a child element item added
to the [children] of the enclosing element item or as the document
element item added to the [children] and [document element] of the
document item. If the element item is a child element item, then the
[parent] is the enclosing element item; otherwise, the [parent] is
the document item.
The [local name] of the element item is the local name of the
expanded name of the NamedType (see [RXEREI]).
Aside: If there are no NAME, ATTRIBUTE-REF, COMPONENT-REF,
ELEMENT-REF, or REF-AS-ELEMENT encoding instructions, then the
local name of the expanded name of a NamedType is the same as the
identifier of the NamedType.
If the namespace name of the expanded name has no value, then the
[namespace name] of the element item has no value (i.e., the
element's name is not namespace qualified); otherwise, the
[namespace name] is the namespace name of the expanded name.
If the type of the NamedType is directly or indirectly the Markup
type, then the [in-scope namespaces] and [namespace attributes] of
the element item are constructed as specified in Section 6.10;
otherwise, the [in-scope namespaces] and [namespace attributes] of
the element item are constructed as specified in Section 6.2.2.1.
If the [namespace name] of the element item has no value, then the
[prefix] of the element item has no value; else if the type of the
NamedType is not directly or indirectly the Markup type, then the
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[prefix] of the element item is determined as specified in
Section 6.2.2.2; otherwise, the [prefix] is determined by the Markup
value as specified in Section 6.10.
The element item becomes the enclosing element item for the
translation of the value of the Type of the NamedType.
For a non-canonical RXER encoding, if the type of the NamedType is
not directly or indirectly the Markup type, then PI and comment items
MAY be added to the [children] of the element item (before or after
any other items). The element item becomes the [parent] for each PI
and comment item. These particular PI and comment items in a
received RXER encoding MAY be discarded by an application.
Aside: There is no provision for representing comments and PIs in
ASN.1 abstract values of types other than the Markup type. These
items will be lost if the abstract value is re-encoded using a
different set of encoding rules.
For a non-canonical RXER encoding, an attribute item with the
[local name] "type" and the [namespace name]
"http://www.w3.org/2001/XMLSchema-instance" (i.e., xsi:type [XSD1])
SHOULD be added to the [attributes] of the element item if the
corresponding NamedType is subject to a TYPE-AS-VERSION encoding
instruction and MAY be added to the [attributes] of the element item
if the Type of the corresponding NamedType is a namespace-qualified
reference (see Section 5). The [prefix] of this attribute item is
determined as specified in Section 6.2.3.1. The [normalized value]
of this attribute item is a qualified name for the expanded name of
the referenced type, with the namespace prefix determined as
specified in Section 6.7.11.1. The element item is the
[owner element] for the attribute item.
Aside: Where a compatible XML Schema translation of the ASN.1
specification has been provided, the xsi:type attribute indicates
to an XML Schema validator which type definition it should use for
validating the RXER encoding.
Aside: An xsi:type attribute is generally not permitted in a CRXER
encoding. Section 6.4 describes some circumstances where it is
required in a CRXER encoding. An xsi:type attribute might also
appear in a CRXER encoding if it is contained in a value of the
Markup type.
For a non-canonical RXER encoding, if the type of the NamedType is
not directly or indirectly the Markup type, then attribute items with
the [local name] "schemaLocation" or "noNamespaceSchemaLocation" and
the [namespace name] "http://www.w3.org/2001/XMLSchema-instance"
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[XSD1] MAY be added to the [attributes] of the element item. The
[prefix] for each of these attribute items is determined as specified
in Section 6.2.3.1. The [normalized value] of these attribute items
MUST reference a compatible XML Schema translation of the ASN.1
specification. The element item is the [owner element] for the
attribute items.
6.2.2.1. Namespace Properties for Elements
This section describes how the [in-scope namespaces] and
[namespace attributes] of an element item are constructed when the
content and attributes of the element item are not described by a
value of the Markup type (otherwise, see Section 6.10).
The [in-scope namespaces] property of the element item initially
contains only the mandatory namespace item for the "xml" prefix
[INFOSET].
For a CRXER encoding, if the element item is not the
[document element] of the document item and the [in-scope namespaces]
property of the element item's [parent] contains a namespace item for
the default namespace, then a namespace declaration attribute item
that undeclares the default namespace (see Section 3) SHALL be added
to the element item's [namespace attributes].
Definition (default namespace restricted): With respect to an element
item, the default namespace is restricted if:
(1) the [namespace name] of the element item has no value (i.e., the
element's name is not namespace qualified), or
(2) the element item is the enclosing element item for a value of the
UNION type where the member attribute will be required (see
Section 6.7.14), or
(3) the element item is the enclosing element item for a value of the
QName type where the namespace-name component is absent (see
Section 6.7.11). This includes the case where the translation of
the QName value is contained in the [normalized value] of an
attribute item in the [attributes] of the element item.
For a non-canonical RXER encoding, if the element item is not the
[document element] of the document item and the [in-scope namespaces]
property of the element item's [parent] contains a namespace item for
the default namespace, then either:
(1) that item is copied to the [in-scope namespaces] of the element
item, or
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(2) a namespace declaration attribute item that declares the default
namespace is added to the element item's [namespace attributes]
(the namespace name is the encoder's choice), and an equivalent
namespace item is added to the [in-scope namespaces] of the
element item, or
(3) a namespace declaration attribute item that undeclares the
default namespace is added to the element item's
[namespace attributes].
Options (1) and (2) SHALL NOT be used if the default namespace is
restricted with respect to the element item.
For a CRXER encoding, if the element item is not the
[document element] of the document item and the element item is not
required to be self-contained, then all the namespace items in the
[in-scope namespaces] of the [parent], excluding the namespace item
for the "xml" prefix and any namespace item for the default
namespace, are copied to the [in-scope namespaces] of the element
item.
For a non-canonical RXER encoding, if the element item is not the
[document element] of the document item and the element item is not
required to be self-contained, then any subset (including none or
all) of the namespace items in the [in-scope namespaces] of the
[parent], excluding certain items, is copied to the
[in-scope namespaces] of the element item. The excluded items that
MUST NOT be copied are: the namespace item for the "xml" prefix, any
namespace item for the default namespace, and any namespace item that
matches the [prefix], but not the [namespace name], of a namespace
item retained for the re-encoding of an unknown attribute item (see
Section 6.8.8) or an unknown alternative of a UNION (see
Section 6.7.14).
Aside: The descriptive approach used by this document only allows
a namespace prefix to be used by a new namespace item if it is not
currently used by another namespace item in the
[in-scope namespaces]. By not inheriting a namespace item, the
prefix of that namespace is again available for reuse without fear
of breaking an existing dependency on the prefix.
Element items that are required to be self-contained inherit none of
the namespace items in the [in-scope namespaces] of the [parent].
Any namespace item that is retained for the re-encoding of an unknown
attribute item (Section 6.8.8) or an unknown alternative of a UNION
(Section 6.7.14) and which is not in the [in-scope namespaces] of the
element item MUST be added to the [in-scope namespaces]. An
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RFC 4910 Robust XML Encoding Rules July 2007
equivalent namespace declaration attribute item MUST be added to the
[namespace attributes] of the element item.
Definition (unused namespace prefix): A namespace prefix is unused if
it does not match the [prefix] of any namespace item in the
[in-scope namespaces] of the element item.
For a non-canonical RXER encoding, if the type of the NamedType is
not directly or indirectly the Markup type, then additional namespace
declaration attribute items for currently unused namespace prefixes
MAY be added to the [namespace attributes] of the element item. An
equivalent namespace item MUST be added to the [in-scope namespaces]
of the element item for each additional namespace declaration
attribute item.
For a non-canonical RXER encoding, if the type of the NamedType is
not directly or indirectly the Markup type, and the
[in-scope namespaces] property of the element item does not contain a
namespace item for the default namespace, and the default namespace
is not restricted with respect to the element item, then a namespace
declaration attribute item for the default namespace MAY be added to
the [namespace attributes] of the element item, in which case an
equivalent namespace item MUST be added to the [in-scope namespaces]
of the element item.
Whenever a namespace declaration attribute item is added to an
element item's [namespace attributes], the [owner element] of the
attribute item is set to the element item.
6.2.2.2. Namespace Prefixes for Element Names
This section describes how the [prefix] of an element item is
determined when the element item has a value for its [namespace name]
and the content and attributes of the element item are not described
by a value of the Markup type (otherwise, see Section 6.10).
For a CRXER encoding, if the [namespace name] of the element item has
a value, then the [prefix] of the element item is any unused
non-canonical namespace prefix unless the [in-scope namespaces]
property of the element item contains a namespace item with the same
[namespace name] as the element item. In that case, the [prefix] of
that namespace item SHALL be used as the [prefix] of the element
item.
Aside: These prefixes will be rewritten to canonical namespace
prefixes during the final step in producing the Infoset
translation (see Section 6.11). Canonical namespace prefixes are
not used here in the first instance because canonicalization
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RFC 4910 Robust XML Encoding Rules July 2007
depends on knowing the final [namespace attributes] produced by
encoding the abstract value of the type of the NamedType. If an
implementation looks ahead to determine this final set prior to
translating the abstract value, then it can assign the appropriate
canonical namespace prefix in this step and skip the rewriting
step.
For a non-canonical RXER encoding, if the [namespace name] has a
value, then the [prefix] of the element item is any unused namespace
prefix unless the [in-scope namespaces] property of the element item
contains a namespace item with the same [namespace name] as the
element item. In that case, the [prefix] of that namespace item MAY
be used as the [prefix] of the element item. Note that the [prefix]
of a namespace item for the default namespace has no value.
If the [prefix] of the element item is an unused namespace prefix,
then a namespace declaration attribute item associating the namespace
prefix with the namespace name MUST be added to the
[namespace attributes] of the element item, and a corresponding
namespace item MUST be added to the [in-scope namespaces] of the
element item.
Aside: The [local name] of the namespace declaration attribute
item is the same as the [prefix] of the element item, the
[namespace name] of the attribute item is
"http://www.w3.org/2000/xmlns/", and the [normalized value] of the
attribute item is the same as the [namespace name] of the element
item. The namespace item has the same [prefix] and
[namespace name] as the element item.
6.2.3. Attribute Components
A value of a NamedType subject to an ATTRIBUTE or ATTRIBUTE-REF
encoding instruction is translated as an attribute item added to the
[attributes] of the enclosing element item (which becomes the
[owner element] of the attribute item).
The [local name] of the attribute item is the local name of the
expanded name of the NamedType (see [RXEREI]).
If the namespace name of the expanded name has no value, then the
[namespace name] of the attribute item has no value; otherwise, the
[namespace name] is the namespace name of the expanded name.
If the [namespace name] has a value, then the [prefix] of the
attribute item is determined as specified in Section 6.2.3.1;
otherwise, the [prefix] of the attribute item has no value.
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The [normalized value] of the attribute item is the translation of
the value of the Type of the NamedType.
For completeness, the [specified] property is set to true, the
[attribute type] has no value, and the value of the [references]
property is set to unknown.
6.2.3.1. Namespace Prefixes for Attribute Names
This section applies when an attribute item with a value for its
[namespace name] is added to the [attributes] of an element item.
For a CRXER encoding, the [prefix] of the attribute item is any
unused non-canonical namespace prefix unless the
[in-scope namespaces] property of the [owner element] contains a
namespace item with a value for the [prefix] (i.e., is not a
namespace item for the default namespace) and the same
[namespace name] as the attribute item. In that case, the [prefix]
of that namespace item SHALL be used as the [prefix] of the attribute
item.
For a non-canonical RXER encoding, the [prefix] of the attribute item
is any unused namespace prefix unless the [in-scope namespaces]
property of the [owner element] contains a namespace item with a
value for the [prefix] and the same [namespace name] as the attribute
item. In that case, the [prefix] of that namespace item MAY be used
as the [prefix] of the attribute item.
If the [prefix] of the attribute item is an unused namespace prefix,
then a namespace declaration attribute item associating the namespace
prefix with the namespace name MUST be added to the
[namespace attributes] of the [owner element], and a corresponding
namespace item MUST be added to the [in-scope namespaces] of the
[owner element].
6.2.4. Unencapsulated Components
A value of a NamedType subject to a GROUP or SIMPLE-CONTENT encoding
instruction is translated as the value of the Type of the NamedType,
i.e., without encapsulation in an element item or attribute item.
Consequently, the enclosing element item for the translation of the
value of the NamedType is also the enclosing element item for the
translation of the value of the Type of the NamedType.
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6.2.5. Examples
Consider this type definition:
CHOICE {
one [0] BOOLEAN,
two [1] [RXER:ATTRIBUTE] INTEGER,
three [2] [RXER:NAME AS "THREE"] OBJECT IDENTIFIER,
four [3] [RXER:ATTRIBUTE-REF {
namespace-name "http://www.example.com",
local-name "foo" }] UTF8String,
five [4] [RXER:ELEMENT-REF {
namespace-name "http://www.example.com",
local-name "bar" }] Markup,
six [5] [RXER:GROUP] SEQUENCE {
seven [0] [RXER:ATTRIBUTE] INTEGER,
eight [1] INTEGER
}
}
The content and attributes of each of the following elements
are the RXER encoding of a value of the above type:
true2.5.4.3another string300
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6.3. Standalone Encodings
A typical RXER encoding is the encoding of a value of a nominated
top-level NamedType. An abstract value MAY be encoded as an XML
document without nominating an explicit top-level NamedType by
invoking a Standalone RXER Encoding or Standalone CRXER Encoding.
In a Standalone RXER Encoding or Standalone CRXER Encoding, the
abstract value is encoded as the value of a notional NamedType where
the identifier of the NamedType is "value" and the Type of the
NamedType is the type of the abstract value. The NamedType is
assumed to be subject to no encoding instructions.
Aside: Thus, the element item corresponding to the document
element will have the [local name] "value" and no value for the
[namespace name] and [prefix].
If RXER is chosen as the transfer syntax in an EMBEDDED PDV value,
then the data-value OCTET STRING SHALL contain a Standalone RXER
encoding.
If CRXER is chosen as the transfer syntax in an EMBEDDED PDV value,
then the data-value OCTET STRING SHALL contain a Standalone CRXER
encoding.
If RXER is chosen as the transfer syntax in an EXTERNAL value, then
the octet-aligned OCTET STRING or arbitrary BIT STRING SHALL contain
a Standalone RXER encoding.
If CRXER is chosen as the transfer syntax in an EXTERNAL value, then
the octet-aligned OCTET STRING or arbitrary BIT STRING SHALL contain
a Standalone CRXER encoding.
6.4. Embedded ASN.1 Values
The reference encoding instructions [RXEREI] allow XML Schema
definitions to be referenced from an ASN.1 specification. It is also
possible to reference an ASN.1 type or top-level NamedType from an
XML Schema definition or from an information item validated by an
XML Schema wildcard. The manner in which an XML Schema definition
references an ASN.1 type or top-level NamedType has an effect on the
CRXER encoding of a value of the type or top-level NamedType.
This section also applies to XML Schema definitions that validate
information items that are contained in a value of the Markup type.
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Aside: So the document element of an XML document might be
described by an XML Schema definition that at some point
references an ASN.1 definition that uses a reference encoding
instruction to reference another XML Schema definition that then
references another ASN.1 definition, and so on.
In each of the following cases, an element or attribute item is only
permitted to be, or to encapsulate, an RXER Infoset translation of an
ASN.1 value if an XML Schema element declaration or ASN.1 NamedType
is known for the [parent] element item ([owner element] in the case
of an attribute declaration), for the [parent] of the [parent]
element item, and so on, to the document element of the XML document.
This condition is not satisfied by a NamedType where the Type is
directly or indirectly the Markup type and the NamedType is not
subject to a reference encoding instruction.
Aside: An element declaration becomes known for an element item
through assessment [XSD1]. A NamedType becomes known for an
element item through decoding.
Aside: If an XML Schema element declaration or ASN.1 NamedType is
not known for an element item, then the type of the element item
and the type of every nested element item are treated as unknown.
Although an xsi:type attribute definitively identifies the type of
an element item even if an element declaration for the element
item is not known, this attribute is generally optional in an RXER
encoding and so cannot be relied upon when seen in isolation from
an element declaration. Although only top-level NamedType
instances can have namespace-qualified names in the current RXER
specification, a future version may allow nested NamedType
instances to also have namespace-qualified names, in which case it
will not necessarily be possible to distinguish a nested NamedType
from a top-level NamedType without knowledge of the type of the
[parent] element item.
An ASN.1 type with an expanded name (Section 5) MAY be referenced by
the type attribute of an XML Schema element declaration. The
reference takes the form of a qualified name for the expanded name.
An element item validated by such an element declaration encapsulates
the Infoset translation of an abstract value of the ASN.1 type. The
[namespace name] and [local name] of the element item are determined
by the XML Schema element declaration. The remaining properties are
determined according to RXER. The element item MUST be
self-contained for a CRXER encoding.
Aside: The element item is not required to be self-contained for a
non-canonical RXER encoding.
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A top-level NamedType MAY be referenced by the ref attribute of an
XML Schema element declaration if the NamedType is not subject to an
ATTRIBUTE encoding instruction. The reference takes the form of a
qualified name for the expanded name of the top-level NamedType
[RXEREI]. An element item validated by such an element declaration
is the Infoset translation of a value of the referenced top-level
NamedType. All the properties of the element item are determined
according to RXER. The element item MUST be self-contained for a
CRXER encoding.
A top-level NamedType MAY be referenced by the ref attribute of an
XML Schema attribute declaration if the NamedType is subject to an
ATTRIBUTE encoding instruction and the definition of the type of the
NamedType does not depend on the QName type in any way. An attribute
item validated by such an attribute declaration is the Infoset
translation of a value of the referenced top-level NamedType, except
that whatever valid [prefix] is initially chosen for the attribute
item MUST be preserved in any re-encoding. The remaining properties
of the attribute item are determined according to RXER.
Aside: The exclusion of the QName type means that the attribute
value is not dependent upon any namespace declarations of its
parent element item.
An element item that is validated by an XML Schema element
declaration that has the ur-type (i.e., anyType) as its type
definition MAY encapsulate the Infoset translation of a value of an
ASN.1 type with an expanded name. The [namespace name] and
[local name] of the element item are determined by the XML Schema
element declaration. The remaining properties of the element item
are determined according to RXER. The [attributes] of the element
item SHALL contain an attribute item with the [local name] "type" and
the [namespace name] "http://www.w3.org/2001/XMLSchema-instance"
(i.e., an xsi:type attribute). The [prefix] of this attribute item
is determined as specified in Section 6.2.3.1. The
[normalized value] of this attribute item is a qualified name for the
expanded name of the ASN.1 type, with the namespace prefix determined
as specified in Section 6.7.11.1. The element item MUST be
self-contained for a CRXER encoding.
An element item that is validated by an XML Schema wildcard (i.e.,
) MAY be the Infoset translation of a value of a top-level
NamedType that is not subject to an ATTRIBUTE encoding instruction
and comes from an ASN.1 module with a target namespace [RXEREI] that
satisfies the namespace constraint of the wildcard. All the
properties of the element item are determined according to RXER. The
element item MUST be self-contained for a CRXER encoding.
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An attribute item that is validated by an XML Schema wildcard (i.e.,
) MAY be the Infoset translation of a value of a
top-level NamedType if the NamedType is subject to an ATTRIBUTE
encoding instruction, comes from an ASN.1 module with a target
namespace that satisfies the namespace constraint of the wildcard,
and has a type that does not depend on the QName type in any way.
Whatever valid [prefix] is initially chosen for the attribute item
MUST be preserved in any re-encoding. The remaining properties of
the attribute item are determined according to RXER.
No other mechanisms for referencing an ASN.1 type or top-level
NamedType from a different XML schema language are supported in this
version of RXER. In particular, this excludes an ASN.1 type being
used as the base type in an XML Schema derivation by extension or
restriction, as a member type for an XML Schema union type, as an
item type for an XML Schema list type, or as the type in an
XML Schema attribute declaration.
A fully conformant RXER implementation will understand both ASN.1 and
XML Schema and will recognize the transitions between information
items controlled by ASN.1 definitions and those controlled by
XML Schema definitions. However, a purely XML Schema validator used
to assess the validity of an RXER encoding will perceive any
reference to an ASN.1 type or top-level NamedType as an unresolved
reference. In order to enable such assessment, it is desirable to
provide an XML Schema translation of the ASN.1 definitions being
referenced from an XML Schema. Although XML Schema and ASN.1 are
broadly similar, they each have unique features that cannot be
adequately expressed in the other language, so a semantically
equivalent translation is not possible in the general case.
Fortunately, to simply achieve successful assessment it is sufficient
for the XML Schema translation of an ASN.1 specification to be
compatible with that ASN.1 specification. That is, the XML Schema
translation MUST be constructed such that every correct RXER encoding
is assessed as valid. Although not ideal, it is acceptable for the
XML Schema to assess some incorrect RXER encodings as also being
valid (a conformant RXER decoder will, of course, reject such an
encoding).
The simplest compatible XML Schema translation of an ASN.1 module is
one in which every type is equivalent to the XML Schema ur-type. For
example, given an ASN.1 type with the reference name MyType, a
sufficient compatible XML Schema type definition is:
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OR
Aside: Because of the possible presence of an asnx:context
attribute (Section 6.8.8.1), it is easiest to assume that all
ASN.1 types translate into XML Schema complex types.
Given an ASN.1 top-level NamedType that is not subject to an
ATTRIBUTE encoding instruction and has the reference name myElement,
a sufficient compatible XML Schema element declaration is:
Given an ASN.1 top-level NamedType that is subject to an ATTRIBUTE
encoding instruction and has the reference name myAttribute, a
sufficient compatible XML Schema attribute declaration is:
An application specification that mixes ASN.1 and XML Schema is free
to provide a stricter translation of its ASN.1 definitions; however,
a more thorough treatment for translating an ASN.1 module into an
XML Schema is out of scope for this document.
6.5. Type Referencing Notations
A value of a type with a defined type name is translated according to
the type definition on the right-hand side of the type assignment for
the type name.
A value of a type denoted by the use of a parameterized type with
actual parameters is translated according to the parameterized type
with the DummyReferences [X.683] substituted with the actual
parameters.
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A value of a constrained type is translated as a value of the type
without the constraint. See X.680 [X.680] and X.682 [X.682] for the
details of ASN.1 constraint notation.
A prefixed type [X.680-1] associates an encoding instruction with a
type. A value of a prefixed type is translated as a value of the
type without the prefix.
Aside: This does not mean that RXER encoding instructions are
ignored. It is simply easier to describe their effects in
relation to specific built-in types, rather than as the
translation of a value of a prefixed type.
A tagged type is a special case of a prefixed type. A value of a
tagged type is translated as a value of the type without the tag.
ASN.1 tags do not appear in the XML encodings defined by this
document.
A value of a fixed type denoted by an ObjectClassFieldType is
translated according to that fixed type (see Section 6.9 for the case
of an ObjectClassFieldType denoting an open type).
A value of a selection type is translated according to the type
referenced by the selection type. Note that component encoding
instructions are not inherited by the type referenced by a selection
type [RXEREI].
A value of a type described by TypeFromObject notation [X.681] is
translated according to the denoted type.
A value of a type described by ValueSetFromObjects notation [X.681]
is translated according to the governing type.
6.6. TypeWithConstraint, SEQUENCE OF Type, and SET OF Type
For the purposes of this document, a TypeWithConstraint is treated as
if it were the parent type [X.680] (either a SEQUENCE OF or SET OF
type).
For example,
SEQUENCE SIZE(1..MAX) OF SomeType
is treated like
SEQUENCE OF SomeType
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Additionally, a "SEQUENCE OF Type" (including the case where it is
the parent type for a TypeWithConstraint) is treated as if it were a
"SEQUENCE OF NamedType", where the identifier of the NamedType is
assumed to be "item". Similarly, a "SET OF Type" (including the case
where it is the parent type for a TypeWithConstraint) is treated as
if it were a "SET OF NamedType", where the identifier of the
NamedType is assumed to be "item".
For example,
SEQUENCE SIZE(1..MAX) OF SomeType
is ultimately treated like
SEQUENCE OF item SomeType
6.7. Character Data Translations
For the majority of ASN.1 built-in types, encodings of values of
those types never have element content. The encoding of a value of
an ASN.1 combining type (except a UNION or LIST type) typically has
element content.
For those types that do not produce element content, the translation
of an abstract value is described as a character string of ISO 10646
characters [UCS]. This character data translation will be destined
to become either part of the [normalized value] of an attribute item,
or a series of character items in the [children] of an element item
(which becomes the [parent] for the character items). The case that
applies is determined in accordance with Section 6.2.
For a non-canonical RXER encoding, if the type of the abstract value
is not directly or indirectly a restricted character string type, the
NULL type, or a UNION type, then leading and/or trailing white space
characters MAY be added to the character data translation.
Aside: White space characters are significant in the encoding of a
value of a restricted character string type, and a restricted
character string type can be a member type of a UNION type. The
encoding of a NULL value produces no character data.
Aside: Optional white space characters are not permitted in a
CRXER encoding.
For a non-canonical RXER encoding, if the type of the abstract value
is directly or indirectly the AnyURI, NCName, or Name type, then
leading and trailing white space characters MAY be added to the
character data translation.
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Aside: These types are indirectly a restricted character string
type (UTF8String); however, their definitions exclude white space
characters, so any white space characters appearing in an encoding
are not part of the abstract value and can be safely ignored.
This exception does not apply to other subtypes of a restricted
character string type that happen to exclude white space
characters.
6.7.1. Restricted Character String Types
The character data translation of a value of a restricted character
string type is the sequence of characters in the string.
Depending on the ASN.1 string type, and an application's internal
representation of that string type, a character may need to be
translated to or from the equivalent ISO 10646 character code [UCS].
The NumericString, PrintableString, IA5String, VisibleString
(ISO646String), BMPString, UniversalString, and UTF8String character
encodings use the same character codes as ISO 10646. For the
remaining string types (GeneralString, GraphicString, TeletexString,
T61String, and VideotexString), see X.680 [X.680].
The null character (U+0000) is not a legal character for XML. It is
omitted from the character data translation of a string value.
Certain other control characters are legal for XML version 1.1, but
not for version 1.0. If any string value contains these characters,
then the RXER encoding must use XML version 1.1 (see Section 6.12).
All white space characters in the RXER encoding of a value of a
restricted character string type (excluding the AnyURI, NCName, and
Name subtypes) are significant, i.e., part of the abstract value.
Examples
The content of each of the following elements is the RXER
encoding of an IA5String value:
Don't run with scissors! Markup (e.g., <value>) has to be escaped.Markup (e.g., ]]>)
has to be escaped.
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6.7.2. BIT STRING
The character data translation of a value of the BIT STRING type is
either a binary digit string, a hexadecimal digit string, or a list
of bit names.
A binary digit string is a sequence of zero, one, or more of the
binary digit characters '0' and '1' (i.e., U+0030 and U+0031). Each
bit in the BIT STRING value is encoded as a binary digit in order
from the first bit to the last bit.
For a non-canonical RXER encoding, if the BIT STRING type has a
NamedBitList, then trailing zero bits MAY be omitted from a binary
digit string.
A hexadecimal digit string is permitted if and only if the number of
bits in the BIT STRING value is zero or a multiple of eight and the
character data translation is destined for the [children] of an
element item.
A hexadecimal digit string is a sequence of zero, one, or more pairs
of the hexadecimal digit characters '0'-'9', 'A'-'F', and 'a'-'f'
(i.e., U+0030-U+0039, U+0041-U+0046 and U+0061-U+0066). Each group
of eight bits in the BIT STRING value is encoded as a pair of
hexadecimal digits where the first bit is the most significant. An
odd number of hexadecimal digits is not permitted. The characters
'a'-'f' (i.e., U+0061-U+0066) SHALL NOT be used in the CRXER encoding
of a BIT STRING value. If a hexadecimal digit string is used, then
the enclosing element's [attributes] MUST contain an attribute item
with the [local name] "format", the [namespace name]
"urn:ietf:params:xml:ns:asnx", and the [normalized value] "hex"
(i.e., asnx:format="hex"). The [prefix] of the attribute item is
determined as specified in Section 6.2.3.1.
Aside: The hexadecimal digit string is intended to conform to the
lexical representation of the XML Schema [XSD2] hexBinary data
type.
For a non-canonical RXER encoding, if the preconditions for using a
hexadecimal digit string are satisfied, then a hexadecimal digit
string MAY be used.
A list of bit names is permitted if and only if the BIT STRING type
has a NamedBitList and each '1' bit in the BIT STRING value has a
corresponding identifier in the NamedBitList.
Aside: ASN.1 does not require that an identifier be assigned for
every bit.
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A list of bit names is a sequence of names for the '1' bits in the
BIT STRING value, in any order, each separated from the next by at
least one white space character. If the BitStringType is not subject
to a VALUES encoding instruction, then each '1' bit in the BIT STRING
value is represented by its corresponding identifier from the
NamedBitList. If the BitStringType is subject to a VALUES encoding
instruction, then each '1' bit in the BIT STRING value is represented
by the replacement name [RXEREI] for its corresponding identifier.
For a CRXER encoding, if the BIT STRING type has a NamedBitList, then
a binary digit string MUST be used, and trailing zero bits MUST be
omitted from the binary digit string; else if the number of bits in
the BIT STRING value is greater than or equal to 64, and the
preconditions for using a hexadecimal digit string are satisfied,
then a hexadecimal digit string MUST be used; otherwise, a binary
digit string MUST be used.
Aside: Because the asnx:format attribute adds an overhead to a
hexadecimal encoding (including a namespace declaration for the
"asnx" prefix), a bit string of less than 64 bits is more
compactly encoded as a binary digit string.
Examples
Consider this type definition:
BIT STRING { black(0), red(1), orange(2), yellow(3),
green(4), blue(5), indigo(6), violet(7) }
The content and attributes of each of the following
elements are an RXER encoding of the same abstract value:
green violet orange 00101001
29
00101001
The final case contains the CRXER encoding of the abstract value.
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6.7.3. BOOLEAN
For a non-canonical RXER encoding, the character data translation of
the BOOLEAN value TRUE is the string "true" or "1", at the encoder's
discretion. For a CRXER encoding, the character data translation of
the BOOLEAN value TRUE is the string "true".
For a non-canonical RXER encoding, the character data translation of
the BOOLEAN value FALSE is the string "false" or "0", at the
encoder's discretion. For a CRXER encoding, the character data
translation of the BOOLEAN value FALSE is the string "false".
Aside: The RXER encoding of BOOLEAN values is intended to conform
to the lexical representation of the XML Schema [XSD2] boolean
data type.
Examples
The content of each of the following elements is the RXER
encoding of a BOOLEAN value:
1
false
false
6.7.4. ENUMERATED
The character data translation of a value of an ENUMERATED type where
the EnumeratedType is not subject to a VALUES encoding instruction is
the identifier corresponding to the actual value.
Examples
Consider this type definition:
ENUMERATED { sunday, monday, tuesday,
wednesday, thursday, friday, saturday }
The content of both of the following elements is the RXER
encoding of a value of the above type:
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monday
thursday
The character data translation of a value of an ENUMERATED type where
the EnumeratedType is subject to a VALUES encoding instruction is the
replacement name [RXEREI] for the identifier corresponding to the
actual value.
Examples
Consider this type definition:
[RXER:VALUES ALL CAPITALIZED,
sunday AS "SUNDAY", saturday AS "SATURDAY"]
ENUMERATED { sunday, monday, tuesday,
wednesday, thursday, friday, saturday }
The content of each of the following elements is the RXER
encoding of a value of the above type:
SUNDAY
Monday
Tuesday
6.7.5. GeneralizedTime
The character data translation of a value of the GeneralizedTime type
is a date, the letter 'T' (U+0054), a time of day, optional
fractional seconds, and an optional time zone.
The date is two decimal digits representing the century, followed by
two decimal digits representing the year, a hyphen ('-', U+002D), two
decimal digits representing the month, a hyphen ('-', U+002D), and
two decimal digits representing the day.
The time of day is two decimal digits representing the hour, followed
by a colon (':', U+003A), two decimal digits representing the
minutes, a colon (':', U+003A), and two decimal digits representing
the seconds.
Note that the hours value "24" is disallowed [X.680].
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A GeneralizedTime value with fractional hours or minutes is first
converted to the equivalent time with whole minutes and seconds and,
if necessary, fractional seconds.
The minutes are encoded as "00" if the GeneralizedTime value omits
minutes. The seconds are encoded as "00" if the GeneralizedTime
value omits seconds.
The fractional seconds part is a full stop ('.', U+002E) followed by
zero, one, or more decimal digits (U+0030-U+0039). For a CRXER
encoding, trailing zero digits (U+0030) in the fractional seconds
SHALL be omitted, and the full stop SHALL be omitted if there are no
following digits.
The time zone, if present, is either the letter 'Z' (U+005A) to
indicate Coordinated Universal Time, a plus sign ('+', U+002B)
followed by a time zone differential, or a minus sign ('-', U+002D)
followed by a time zone differential.
A time zone differential indicates the difference between local time
(the time specified by the preceding date and time of day) and
Coordinated Universal Time. Coordinated Universal Time can be
calculated from the local time by subtracting the differential.
For a CRXER encoding, a GeneralizedTime value with a time zone
differential SHALL be encoded as the equivalent Coordinated Universal
Time, i.e., the time zone will be "Z".
A local time GeneralizedTime value is not converted to Coordinated
Universal Time for a CRXER encoding. Other canonical ASN.1 encoding
rules specify that local times must be encoded as Coordinated
Universal Time but do not specify a method to convert a local time to
a Coordinated Universal Time. Consequently, canonicalization of
local time values is unreliable and applications SHOULD NOT use local
time.
A time zone differential is encoded as two decimal digits
representing hours, a colon (':', U+003A), and two decimal digits
representing minutes. The minutes are encoded as "00" if the
GeneralizedTime value omits minutes from the time zone differential.
Aside: The RXER encoding of GeneralizedTime values is intended to
conform to the lexical representation of the XML Schema [XSD2]
dateTime data type.
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Examples
The content of each of the following elements is the RXER
encoding of a GeneralizedTime value:
2004-06-15T12:00:00Z 2004-06-15T02:00:00+10:00
2004-06-15T12:00:00.5
6.7.6. INTEGER
For a CRXER encoding, the character data translation of a value of an
IntegerType is a canonical number string representing the integer
value.
A canonical number string is either the digit character '0' (U+0030),
or an optional minus sign ('-', U+002D) followed by a non-zero
decimal digit character (U+0031-U+0039) followed by zero, one, or
more of the decimal digit characters '0' to '9' (U+0030-U+0039).
For a non-canonical RXER encoding, the character data translation of
a value of the IntegerType without a NamedNumberList is a number
string representing the integer value.
A number string is a sequence of one or more of the decimal digit
characters '0' to '9' (U+0030-U+0039), with an optional leading sign,
either '+' (U+002B) or '-' (U+002D). Leading zero digits are
permitted in a number string for a non-canonical RXER encoding.
Aside: The RXER encoding of values of the IntegerType without a
NamedNumberList is intended to conform to the lexical
representation of the XML Schema [XSD2] integer data type.
For a non-canonical RXER encoding, if the IntegerType has a
NamedNumberList, and the NamedNumberList defines an identifier for
the actual value, and the IntegerType is not subject to a VALUES
encoding instruction, then the character data translation of the
value is either a number string or the identifier.
Examples
Consider this type definition:
INTEGER { zero(0), one(1) }
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The content of each of the following elements is the RXER
encoding of a value of the above type:
0 zero 2 00167
For a non-canonical RXER encoding, if the IntegerType is subject to a
VALUES encoding instruction (it necessarily must have a
NamedNumberList) and the NamedNumberList defines an identifier for
the actual value, then the character data translation of the value is
either a number string or the replacement name [RXEREI] for the
identifier.
Examples
Consider this type definition:
[RXER:VALUES ALL UPPERCASED] INTEGER { zero(0), one(1) }
The content of both of the following elements is the RXER
encoding of a value of the above type:
0 ZERO
6.7.7. NULL
The character data translation of a value of the NULL type is an
empty character string.
Examples
The final case is the CRXER encoding.
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6.7.8. ObjectDescriptor
A value of the ObjectDescriptor type is translated according to the
GraphicString type.
6.7.9. OBJECT IDENTIFIER and RELATIVE-OID
The character data translation of a value of the OBJECT IDENTIFIER or
RELATIVE-OID type is a full stop ('.', U+002E) separated list of the
object identifier components of the value.
Each object identifier component is translated as a non-negative
number string. A non-negative number string is either the digit
character '0' (U+0030), or a non-zero decimal digit character
(U+0031-U+0039) followed by zero, one, or more of the decimal digit
characters '0' to '9' (U+0030-U+0039).
Examples
The content of each of the following elements is the RXER
encoding of an OBJECT IDENTIFIER value:
2.5.6.0
2.5.4.10
2.5.4.3
6.7.10. OCTET STRING
The character data translation of a value of the OCTET STRING type is
the hexadecimal digit string representation of the octets.
The octets are encoded in order from the first octet to the last
octet. Each octet is encoded as a pair of the hexadecimal digit
characters '0'-'9', 'A'-'F', and 'a'-'f' (i.e., U+0030-U+0039,
U+0041-U+0046, and U+0061-U+0066) where the first digit in the pair
corresponds to the four most significant bits of the octet. An odd
number of hexadecimal digits is not permitted. The characters 'a'-
'f' (i.e., U+0061-U+0066) SHALL NOT be used in the CRXER encoding of
an OCTET STRING value.
Aside: The RXER encoding of OCTET STRING values is intended to
conform to the lexical representation of the XML Schema [XSD2]
hexBinary data type.
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Examples
The content of each of the following elements is the RXER
encoding of an OCTET STRING value:
27F69A0300
efA03bFF
6.7.11. QName
The character data translation of a value of the QName type
(Section 4.5) is a qualified name conforming to the QName production
of Namespaces in XML 1.0 [XMLNS10].
The local part (i.e., LocalPart) of the qualified name SHALL be the
value of the local-name component of the QName value.
If the namespace-name component of the QName value is absent, then
the namespace prefix (i.e., Prefix) of the qualified name SHALL be
absent; otherwise, the namespace prefix is determined as specified in
Section 6.7.11.1 using the value of the namespace-name component of
the QName value as the namespace name.
6.7.11.1. Namespace Prefixes for Qualified Names
This section describes how the namespace prefix of a qualified name
is determined given the namespace name to which the namespace prefix
must map.
For a CRXER encoding, the namespace prefix of the qualified name is
any unused non-canonical namespace prefix unless the
[in-scope namespaces] property of the enclosing element item contains
a namespace item with a [namespace name] that matches the namespace
name. In that case, the [prefix] of that namespace item SHALL be
used as the namespace prefix of the qualified name.
Aside: If the qualified name appears in the [normalized value] of
an attribute item, then the enclosing element item is the
[owner element] for that attribute item.
For a non-canonical RXER encoding, the namespace prefix of the
qualified name is any unused namespace prefix unless the
[in-scope namespaces] property of the enclosing element item contains
a namespace item with the same [namespace name] as the element item.
In that case, the [prefix] of that namespace item MAY be used as the
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namespace prefix of the qualified name. Note that the [prefix] of a
namespace item for the default namespace has no value.
If the namespace prefix of the qualified name is an unused namespace
prefix, then a namespace declaration attribute item associating the
namespace prefix with the namespace name MUST be added to the
[namespace attributes] of the enclosing element item, and a
corresponding namespace item MUST be added to the
[in-scope namespaces] of the enclosing element item.
6.7.12. REAL
The character data translation of a value of the REAL type is the
character string "0" if the value is positive zero, the character
string "-0" if the value is negative zero, the character string "INF"
if the value is positive infinity, the character string "-INF" if the
value is negative infinity, the character string "NaN" if the value
is not a number, or a real number otherwise.
A real number is the mantissa followed by either the character 'E'
(U+0045) or 'e' (U+0065) and the exponent. The character 'e' SHALL
NOT be used for a CRXER encoding. If the exponent is zero, then the
'E' or 'e' and exponent MAY be omitted for a non-canonical RXER
encoding.
The mantissa is a decimal number with an optional leading sign,
either '+' (U+002B) or '-' (U+002D). A decimal number is a sequence
of one or more of the decimal digit characters '0' to '9'
(U+0030-U+0039) optionally partitioned by a single full stop
character ('.', U+002E) representing the decimal point. Multiple
leading zero digits are permitted for a non-canonical RXER encoding.
The exponent is encoded as a number string (see Section 6.7.6).
Aside: The RXER encoding of REAL values is intended to be
compatible with the lexical representation of the XML Schema
[XSD2] double data type, but allows real values outside the set
permitted by double.
For a CRXER encoding:
(1) The real number MUST be normalized so that the mantissa has a
single non-zero digit immediately to the left of the decimal
point.
(2) Leading zero digits SHALL NOT be used.
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(3) A leading plus sign SHALL NOT be used in the mantissa or the
exponent.
(4) The fractional part of the mantissa (i.e., that part following
the decimal point) MUST have at least one digit (which may be
'0') and MUST NOT have any trailing zeroes after the first digit.
(5) The exponent SHALL be present and SHALL be a canonical number
string (see Section 6.7.6).
Examples
The content of each of the following elements is the RXER
encoding of a REAL value:
3.14159 1.0e6 INF
-01e-06
6.7.13. UTCTime
The character data translation of a value of the UTCTime type is a
date, the letter 'T' (U+0054), a time of day, and a time zone.
The date is two decimal digits representing the year (no century), a
hyphen ('-', U+002D), two decimal digits representing the month, a
hyphen ('-', U+002D), and two decimal digits representing the day.
The time of day is two decimal digits representing the hour, followed
by a colon (':', U+003A), two decimal digits representing the
minutes, a colon (':', U+003A), and two decimal digits representing
the seconds.
Note that the hours value "24" is disallowed [X.680].
The seconds are encoded as "00" if the UTCTime value omits seconds.
The time zone is either the letter 'Z' (U+005A) to indicate
Coordinated Universal Time, a plus sign ('+', U+002B) followed by a
time zone differential, or a minus sign ('-', U+002D) followed by a
time zone differential.
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A time zone differential indicates the difference between local time
(the time specified by the preceding date and time of day) and
Coordinated Universal Time. Coordinated Universal Time can be
calculated from the local time by subtracting the differential.
For a CRXER encoding, a UTCTime value with a time zone differential
SHALL be encoded as the equivalent Coordinated Universal Time, i.e.,
the time zone will be "Z".
A time zone differential is encoded as two decimal digits
representing hours, a colon (':', U+003A), and two decimal digits
representing minutes.
6.7.14. CHOICE as UNION
The chosen alternative of a value of a UNION type corresponds to some
NamedType in the UNION type definition (a ChoiceType).
The character data translation of a value of a UNION type is the
character data translation of the value of the type of the chosen
alternative, i.e., without any kind of encapsulation.
Leading and trailing white space characters are not permitted to be
added to the character data translation of a value of a UNION type
(see Section 6.7); however, this does not preclude such white space
being added to the character data translation of the value of the
chosen alternative.
The character data translation of a value of a UNION type is
necessarily destined for the [children] of an enclosing element item.
Aside: This is because the ATTRIBUTE encoding instruction cannot
be applied to a NamedType with a type that is a UNION type.
The chosen alternative can be identified by a member attribute item,
i.e., an attribute item with the [local name] "member" and
[namespace name] "urn:ietf:params:xml:ns:asnx", added to the
[attributes] of the enclosing element item. The [prefix] of this
attribute item is determined as specified in Section 6.2.3.1. The
[normalized value] of the attribute item is a qualified name for the
expanded name of the NamedType (see [RXEREI]) corresponding to the
chosen alternative.
Aside: It is not possible to associate a namespace name with a
NamedType in a UNION type using the current specification for RXER
encoding instructions. Consequently, the [normalized value] of
the member attribute item will always contain a qualified name
without a namespace prefix.
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For a CRXER encoding, the member attribute item MUST be used, and the
[normalized value] of the attribute item MUST be the CRXER
translation of the QName value equal to the expanded name.
In the absence of a member attribute item, an RXER decoder MUST
determine the chosen alternative by considering the alternatives of
the choice in the order prescribed below and accepting the first
alternative for which the encoding is valid.
If the UNION encoding instruction has a PrecedenceList, then the
alternatives of the ChoiceType referenced by the PrecedenceList are
considered in the order identified by that PrecedenceList, then the
remaining alternatives are considered in the order of their
definition in the ChoiceType. If the UNION encoding instruction does
not have a PrecedenceList, then all the alternatives of the
ChoiceType are considered in the order of their definition in the
ChoiceType.
A non-canonical RXER encoder MUST use the member attribute item if an
RXER decoder would determine the chosen alternative to be something
other than the actual chosen alternative of the CHOICE value being
translated; otherwise, the member attribute item MAY be used.
Examples
Consider this type definition:
[RXER:UNION PRECEDENCE serialNumber] CHOICE {
name [0] IA5String,
serialNumber [1] INTEGER
}
In the absence of a member attribute, an RXER decoder would first
consider whether the received encoding was a valid serialNumber
(an INTEGER) before considering whether it was a valid name (an
IA5String).
The content and attributes of each of the following
elements are the RXER encoding of a value of the above type:
BobAlice
344
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100
The member attribute is required in the final case to prevent the
value being interpreted as a serialNumber.
If the UNION (i.e., CHOICE) type is extensible [X.680], then an
application MUST accept and be prepared to re-encode (using the same
encoding rules) any unknown extension in received encoded values of
the type. An unknown extension in a value of a UNION type (an
unknown alternative) takes the form of an unknown name in the
[normalized value] of the member attribute and/or character data in
the [children] of the enclosing element item that do not conform to
any of the known alternatives.
To enable re-encoding of an unknown alternative, it is necessary to
retain the [normalized value] of the member attribute, if present,
and the [children] property of the enclosing element item.
The character data for an unknown alternative may contain qualified
names that depend on the [in-scope namespaces] of the enclosing
element item for their interpretation. Therefore, semantically
faithful re-encoding of an unknown alternative may require
reproduction of at least some part of the [in-scope namespaces] of
the enclosing element item. The problem is deciding which of the
namespace items are actually needed. In the absence of type
information, it is not possible to discern whether anything that
syntactically resembles a qualified name in the character data of the
enclosing element item actually is a qualified name. The simplest
approach is to retain all the namespace items from the
[in-scope namespaces] of the enclosing element item and output them
as namespace declaration attribute items in the
[namespace attributes] of the enclosing element item when re-encoding
the unknown alternative. At best, an application can omit the
namespace items that do not define the namespace prefix of any
potential qualified name.
An application MUST retain the namespace items in the
[in-scope namespaces] of the enclosing element item that define the
namespace prefixes of all the potential qualified names in the
[children] of the enclosing element item. Other namespace items in
the [in-scope namespaces] of the enclosing element item MAY be
retained. The effect of these retained namespace items on the
[namespace attributes] and [in-scope namespaces] of the enclosing
element item when re-encoding is considered in Section 6.2.2.1.
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Aside: The context attribute (Section 6.8.8) is not added to the
[attributes] of the enclosing element item when re-encoding an
unknown alternative since the type of a NamedType in a UNION type
cannot be the Markup type.
6.7.15. SEQUENCE OF as LIST
The character data translation of a value of a LIST type (a
SEQUENCE OF NamedType) is the concatenation of the character data
translations of the component values, i.e., the abstract values of
the type of the NamedType, each separated from the next by at least
one white space character. For a CRXER encoding, separating white
space MUST be exactly one space character (U+0020).
Example
Consider this type definition:
[LIST] SEQUENCE OF timeStamp GeneralizedTime
The content of the following element is the RXER encoding
of a value of the above type:
2004-06-15T12:14:56Z
2004-06-15T12:18:13Z
2004-06-15T01:00:25Z
6.8. Combining Types
The encoding of a value of an ASN.1 combining type (except a UNION or
LIST type) typically has element content.
The Infoset translation of a value of a specific ASN.1 combining type
(excluding a UNION or LIST type) contains zero or more attribute
items to be added to the [attributes] of the enclosing element item
and zero or more element items to be added to the [children] of the
enclosing element item. These translations are described in Sections
6.8.1 to 6.8.7.
For a non-canonical RXER encoding, white space character items MAY be
added to the [children] of the enclosing element item (before or
after any other items).
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For a CRXER encoding, a character item with the [character code]
U+000A (a line feed) MUST be inserted immediately before each element
item in the [children] of the enclosing element item. No other white
space character items are permitted to be added to the [children] of
the enclosing element item.
Aside: Without the single line feed character before each child
element, a typical CRXER encoding would be a single, very long
line.
6.8.1. CHARACTER STRING
A value of the unrestricted CHARACTER STRING type is translated
according to the corresponding SEQUENCE type defined in Clause 40.5
of X.680 [X.680].
6.8.2. CHOICE
The chosen alternative of a value of a CHOICE type corresponds to,
and is a value of (see Section 6), some NamedType in the CHOICE type
definition.
The translation of a value of a CHOICE type other than the Markup
type or a UNION type (see Section 6.7.14) is the translation of the
value of the NamedType corresponding to the actual chosen
alternative.
Examples
Consider this type definition:
CHOICE {
name [0] IA5String,
serialNumber [1] INTEGER
}
The content of each of the following elements is the RXER
encoding of a value of the above type:
BobAlice
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344
100
If the CHOICE type is extensible [X.680], then an application MUST
accept, and be prepared to re-encode (in RXER), any attribute item or
child element item with a name that is not recognized (see
Section 6.8.8).
6.8.3. EMBEDDED PDV
A value of the EMBEDDED PDV type is translated according to the
corresponding SEQUENCE type defined in Clause 33.5 of X.680 [X.680].
6.8.4. EXTERNAL
A value of the EXTERNAL type is translated according to the
corresponding SEQUENCE type defined in Clause 8.18.1 of X.690
[X.690].
6.8.5. INSTANCE OF
A value of the INSTANCE OF type is translated according to the
corresponding SEQUENCE type defined in Annex C of X.681 [X.681].
6.8.6. SEQUENCE and SET
Each component value of a value of a SEQUENCE or SET type corresponds
to, and is a value of (see Section 6), some NamedType in the SEQUENCE
or SET type definition.
A value of a SEQUENCE or SET type, other than the QName type
(Section 4.5), is translated by translating in turn each component
value actually present in the SEQUENCE or SET value and adding the
resulting attribute items and/or element items to the [attributes]
and/or [children] of the enclosing element item. Attribute items may
be added to the [attributes] of the enclosing element item in any
order. Element items resulting from the translation of component
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values MUST be appended to the [children] of the enclosing element
item in the order of the component values' corresponding NamedType
definitions in the SEQUENCE or SET type definition.
Aside: In the case of the SET type, this is a deliberate departure
from BER [X.690], where the components of a SET can be encoded in
any order.
If a DEFAULT value is defined for a NamedType and the value of the
NamedType is the same as the DEFAULT value, then the translation of
the value of the NamedType SHALL be omitted for a CRXER encoding and
MAY be omitted for a non-canonical RXER encoding.
Examples
Consider this type definition:
SEQUENCE {
name [0] IA5String OPTIONAL,
partNumber [1] INTEGER,
quantity [2] INTEGER DEFAULT 0
}
The content of each of the following elements is the RXER
encoding of a value of the above type:
23chisel 37 0 154329
If the SEQUENCE or SET type is extensible [X.680], then an
application MUST accept, and be prepared to re-encode (in RXER), any
attribute item or child element item with a name that is not
recognized (see Section 6.8.8).
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6.8.7. SEQUENCE OF and SET OF
Each component value of a value of a type that is a SET OF NamedType
or a SEQUENCE OF NamedType corresponds to, and is a value of (see
Section 6), the NamedType in the type definition.
A value of a type that is a SET OF NamedType, or a
SEQUENCE OF NamedType other than a LIST type (see Section 6.7.15), is
translated by adding the translation of each value of the NamedType
to the [children] of the enclosing element item.
Aside: An ATTRIBUTE encoding instruction cannot appear in the
component type for a SEQUENCE OF or SET OF type, so there are no
attribute items to add to the [attributes] of the enclosing
element item.
If the type is a SEQUENCE OF NamedType, then the values of the
NamedType are translated in the order in which they appear in the
value of the type.
For a non-canonical RXER encoding, if the type is a SET OF NamedType,
then the values of the NamedType may be translated in any order.
For a CRXER encoding, if the type is a SET OF NamedType, then the
values of the NamedType MUST be translated in ascending order where
the order is determined by comparing the octets of their CRXER
encodings (which will be UTF-8 encoded character strings; see
Section 6.12.2). A shorter encoding is ordered before a longer
encoding that is identical up to the length of the shorter encoding.
Examples
Consider this type definition:
SEQUENCE OF timeStamp GeneralizedTime
The content of the following element is the RXER encoding
of a value of the above type:
2004-06-15T12:14:56Z2004-06-15T12:18:13Z
2004-06-15T01:00:25Z
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Consider this type definition (also see Section 6.6):
SEQUENCE OF INTEGER
The content of the following element is the RXER encoding
of a value of the above type:
12
9
7
6.8.8. Extensible Combining Types
An application must accept and be prepared to re-encode (using the
same encoding rules) any unknown extension appearing in the encoding
of a value of an extensible CHOICE, SEQUENCE, or SET type. An
unknown extension in a value of an extensible combining type (except
UNION types) takes the form of unknown element and/or attribute
items. Section 6.8.8.1 describes the processing of unknown element
items and Section 6.8.8.2 describes the processing of unknown
attribute items.
An application cannot produce a canonical encoding if an abstract
value contains unknown extensions. However, the method for
re-encoding unknown extensions does not prevent a receiving
application with knowledge of the extension from producing the
correct canonical encoding.
6.8.8.1. Unknown Elements in Extensions
To enable re-encoding of an unknown element item it is necessary to
retain the [prefix], [local name], [attributes],
[namespace attributes], and [children] properties of the element
item.
Definition (inherited namespace item): An inherited namespace item
is a namespace item in the [in-scope namespaces] of an element item
for which there is no corresponding namespace declaration attribute
item in the [namespace attributes] of the element item.
The content and attributes of an unknown element item may contain
qualified names whose interpretation depends on inherited namespace
items. Semantically faithful re-encoding of the unknown item may
require reproduction of at least some of the inherited namespace
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items. The problem is deciding which of the inherited namespace
items are actually needed. Qualified names as the names of elements
and attributes are easily recognized, but in the absence of type
information it is not possible to discern whether anything that
syntactically resembles a qualified name in the value of an attribute
or the character data of an element actually is a qualified name.
The simplest approach is to retain all the inherited namespace items
and output corresponding namespace declaration attribute items in the
[namespace attributes] of the unknown element item when re-encoding
the element item. At best, an application can omit the inherited
namespace items that do not define the namespace prefix of any
definite or potential qualified name, though this requires examining
the content and attributes of the unknown extension.
Regardless of how clever an implementation tries to be, adding any
namespace declaration attribute items to an unknown element item is
harmful to canonicalization if the ASN.1 type for the element item
turns out to be the Markup type. To counter this problem, a special
attribute is used to identify the namespace declaration attribute
items added to an unknown element item so that they can be removed
later, if it proves necessary.
If the outermost element item in an unknown extension does not have
an attribute item with the [local name] "context" and
[namespace name] "urn:ietf:params:xml:ns:asnx" in its [attributes],
then namespace declaration attribute items corresponding to the
inherited namespace items that define the namespace prefixes of all
the definite and potential qualified names in the content and
attributes of the element item MUST be added to the retained
[namespace attributes]. Other inherited namespace items MAY be added
to the retained [namespace attributes].
If there are one or more of these added namespace declaration
attribute items, then an attribute item with the [local name]
"context" and [namespace name] "urn:ietf:params:xml:ns:asnx" MUST be
added to the retained [attributes].
The [prefix] of the context attribute item is any namespace prefix
that does not match the [local name] of any namespace declaration
attribute item in the [namespace attributes] unless the
[namespace attributes] property contains a namespace declaration
attribute item with a non-empty [prefix] and a [normalized value] of
"urn:ietf:params:xml:ns:asnx". In that case, the [local name] of
that namespace declaration attribute item MAY be used as the [prefix]
of the context attribute item.
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If the [prefix] of the context attribute item does not match the
[local name] of any namespace declaration attribute item, then an
attribute item with the [prefix] "xmlns", [namespace name]
"urn:ietf:params:xml:ns:asnx", and [local name] equal to the [prefix]
of the context attribute item MUST be added to the retained
[namespace attributes] of the element item.
The [normalized value] of the context attribute is the white-space-
separated unordered list of the [local names] of the added namespace
declaration attribute items (i.e., a list of the namespace prefixes),
including any namespace declaration attribute item added to define
the [prefix] of the context attribute. Note that the [local name]
for a namespace declaration attribute item declaring the default
namespace is "xmlns".
Aside: A receiver that knows about the extension will use the
context attribute to strip out the added namespace declaration
attributes if the type of the associated NamedType is the Markup
type (Section 6.10), and will discard the context attribute
otherwise. A receiver that does not know about the extension will
re-encode the extension as is.
Adding the required namespace declaration attribute items to an
element item effectively makes the element item self-contained. A
received encoding has an encoding error if it contains an element
item that is not self-contained but has a context attribute item in
its [attributes].
An RXER encoder MUST NOT add the context attribute item to an element
item corresponding to a NamedType that is known to it.
An RXER decoder MUST accept the context attribute item on an element
item corresponding to a NamedType that does not appear to be an
extension.
Aside: It is not uncommon for extension markers to be neglected in
specifications traditionally using only BER, since extension
markers do not alter BER encodings. Consequently, it is not
immediately obvious in later versions of the specification which
instances of NamedType belong to extensions of the original base
specification.
Example
Suppose there are three applications, A, B, and C. Suppose that
Application A uses the first edition of an ASN.1 specification
containing the following type definition:
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MyType ::= SEQUENCE {
field1 INTEGER, -- present in first edition
...
}
Suppose that Application B uses the second edition of the ASN.1
specification:
MyType ::= SEQUENCE {
field1 INTEGER, -- present in first edition
...,
field2 QName -- added in second edition
}
Suppose that Application C uses the third edition of the ASN.1
specification:
MyType ::= SEQUENCE {
field1 INTEGER, -- present in first edition
...,
field2 QName, -- added in second edition
field3 Markup -- added in third edition
}
Application C produces the following RXER encoding and sends it to
Application B:
100 p2:foobar p1:foobar
Application B doesn't know about , so it adds the
asnx:context attribute to when it re-encodes the abstract
value to send to Application A:
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100p1:foobar p1:foobar
Application A doesn't know about and , so it adds
the asnx:context attribute to and leaves alone
when it re-encodes the abstract value:
100 p1:foobar p1:foobar
If Application C receives this final encoding, it has sufficient
information to discard the asnx:context, xmlns:asnx, and xmlns:p2
attributes from the received Markup value of to recover
the original value. Application C knows about , so it
uses the namespace declaration for p1 when decoding the QName
value and ignores the other declarations.
6.8.8.2. Unknown Attributes in Extensions
To enable re-encoding of an unknown attribute item it is necessary to
retain at least the [local name], [namespace name], and
[normalized value] properties of the attribute item.
The [normalized value] of an unknown attribute item may contain
qualified names whose interpretation depends on the
[in-scope namespaces] of the [owner element]. Semantically faithful
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re-encoding of the unknown attribute item may require reproduction of
at least some part of the [in-scope namespaces]. In the absence of
type information, it is not possible to discern whether anything that
syntactically resembles a qualified name in the [normalized value] of
an unknown attribute item actually is a qualified name.
The simplest approach is to retain all the namespace items of the
[in-scope namespaces] and output corresponding namespace declaration
attribute items in the [namespace attributes] of the [owner element]
when re-encoding the extension. At best, an application can omit the
namespace items that do not define the namespace prefix of any
potential qualified name in the [normalized value].
An application MUST retain the namespace items in the
[in-scope namespaces] of the [owner element] that define the
namespace prefixes of all the potential qualified names in the
[normalized value] of the unknown attribute item. Other namespace
items in the [in-scope namespaces] of the [owner element] MAY be
retained.
Aside: If the enclosing element item has more than one unknown
attribute item, then it is sufficient to save the union of the
retained namespace items with the element item, rather than saving
the retained namespace items with each unknown attribute item.
When the unknown attribute item is re-encoded, the retained namespace
items affect the [namespace attributes] and [in-scope namespaces] of
the enclosing element item as specified in Section 6.2.2.1, and the
[prefix] of the attribute item is determined as specified in
Section 6.2.3.1.
Aside: The context attribute is not added to the [attributes] of
the [owner element] when re-encoding an unknown attribute item
because the type of a NamedType subject to an ATTRIBUTE or
ATTRIBUTE-REF encoding instruction cannot be the Markup type.
6.9. Open Type
A value of an open type denoted by an ObjectClassFieldType [X.681] is
translated according to the specific Type of the value.
If the specific Type of the value is directly or indirectly the
Markup type, then the enclosing element item MUST be self-contained.
For a non-canonical RXER encoding, if the translation of the value
does not generate an attribute item with the [local name] "type" and
the [namespace name] "http://www.w3.org/2001/XMLSchema-instance"
(i.e., xsi:type) and the specific Type of the value is a
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namespace-qualified reference (Section 5), then an attribute item
with the [local name] "type" and the [namespace name]
"http://www.w3.org/2001/XMLSchema-instance" (i.e., xsi:type) MAY be
added to the [attributes] of the enclosing element item. The
[normalized value] of this attribute item is a qualified name for the
expanded name of the referenced type, with the namespace prefix
determined as specified in Section 6.7.11.1.
Aside: The xsi:type attribute is added by RXER encoders for the
benefit of XML Schema validators. This attribute tells an
XML Schema validator which type definition in a compatible
XML Schema translation of the ASN.1 specification it should use
for validating the content and attributes of the enclosing
element. For an RXER decoder, the actual type in an open type
value is generally determined by an associated component relation
constraint [X.682], so the xsi:type attribute can be ignored.
Example
The content and attributes of the following element are
the RXER encoding of an open type value containing a BOOLEAN
value:
true
If the ObjectClassFieldType denoting an open type is not constrained
by a TableConstraint, or is constrained by a TableConstraint where
the constraining object set is extensible, then an application MUST
accept and be prepared to re-encode (using the same encoding rules)
any value of the open type where the specific Type of the value is
unknown. In such cases, the enclosing element item is treated like
an unknown element item in the value of an extensible combining ASN.1
type (see Section 6.8.8.1).
6.10. Markup
Conceptually, a value of the Markup type holds the [prefix],
[attributes], [namespace attributes], and [children] of an element
item. The Infoset translation of a value of the Markup type
initially simply sets the [prefix], [attributes],
[namespace attributes], and [children] of the enclosing element item
to the corresponding properties represented by the Markup value.
Recall that the enclosing element item for the translation of a
Markup value is required to be self-contained (Section 4.1.1).
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If the enclosing element item is not the [document element] of the
document item, and the [in-scope namespaces] property of the
enclosing element item's [parent] contains a namespace item for the
default namespace, and the [namespace attributes] property
represented by the Markup value does not contain a namespace item
declaring or undeclaring the default namespace, then a namespace
declaration attribute item that undeclares the default namespace
SHALL be added to the enclosing element item's
[namespace attributes].
It is not necessary to populate the [in-scope namespaces] of the
enclosing element item for encoding purposes (though it may be
warranted for other purposes).
An element item nested in the [children] is potentially the Infoset
translation of a value of a top-level NamedType (as allowed by
Section 6.4), and the entire Markup value can represent the content
and attributes of an element item that is the translation of a value
of a top-level NamedType.
Aside: The latter case arises when an ELEMENT-REF encoding
instruction references a top-level NamedType.
The content and attributes of an element item nested in the
[children] of a Markup value are potentially the Infoset translation
of an abstract value of an ASN.1 type (as allowed by Section 6.4),
and the entire Markup value can represent the translation of a single
abstract value.
Aside: The latter case arises when a TYPE-REF encoding instruction
references an ASN.1 type.
For a non-canonical RXER encoding, any element item, at any level of
nesting (including the enclosing element item itself), that
corresponds to the value of a top-level NamedType MAY be replaced
with any valid translation of that value.
For a non-canonical RXER encoding, any element item, at any level of
nesting (including the enclosing element item itself), with content
and attributes that correspond to an abstract value of an ASN.1 type
MAY have that content and those attributes replaced with any valid
translation of that abstract value. If the content and attributes
are replaced, then the [prefix], [in-scope namespaces], and
[namespace attributes] of the element item are constructed as
specified in Sections 6.2.2.1 and 6.2.2.2. The enclosing element
item for the Markup value is still required to be self-contained.
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Aside: Insofar as a Markup value represents ASN.1 abstract values,
it is sufficient for the RXER encoding of the Markup value to
preserve the abstract values rather than preserve the exact
Infoset representation.
For a CRXER encoding, any element item, at any level of nesting
(including the enclosing element item itself), that corresponds to a
value of a top-level NamedType MUST be replaced with the CRXER
translation of that value.
For a CRXER encoding, any element item, at any level of nesting
(including the enclosing element item itself), with content and
attributes that correspond to an abstract value of an ASN.1 type MUST
have that content and those attributes replaced with the CRXER
translation of that abstract value. The [prefix],
[in-scope namespaces], and [namespace attributes] of the element item
are constructed as specified in Sections 6.2.2.1 and 6.2.2.2.
If the [attributes] property of the enclosing element item from a
received RXER encoding contains an attribute item with the
[local name] "context" and [namespace name]
"urn:ietf:params:xml:ns:asnx" (i.e., asnx:context), then this
attribute item MUST be omitted from the [attributes] represented by
the Markup value, and each namespace declaration attribute item with
a [local name] matching an NCName in the [normalized value] of the
attribute item MUST be omitted from the [namespace attributes]
represented by the Markup value.
6.11. Namespace Prefixes for CRXER
The final step in translating the value of a top-level NamedType for
a CRXER encoding, or an abstract value for a Standalone CRXER
Encoding, is the replacement of the arbitrarily chosen namespace
prefixes with algorithmically determined canonical namespace
prefixes. This procedure for prefix replacement applies to each
element item where the [namespace attributes] have been constructed
according to Section 6.2.2.1. This includes any element item
corresponding to a value of a top-level NamedType, or with content
and attributes that correspond to an abstract value of an ASN.1 type,
that is nested in a value of the Markup type.
For each element item where prefix replacement applies, the following
sequence of steps is repeated until there are no more eligible
attribute items to select in step (1):
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RFC 4910 Robust XML Encoding Rules July 2007
(1) Select the attribute item with the least [normalized value] from
amongst the attribute items of the [namespace attributes] that
have a [local name] that is not a canonical namespace prefix
(i.e., select from the namespace declaration attribute items that
have not already been processed). A [normalized value] is less
than another [normalized value] if the former appears before the
latter in an ordering of the values determined by comparing the
ISO 10646 code points [UCS] of their characters, from first to
last. A shorter string of characters is ordered before a longer
string of characters that is identical up to the length of the
shorter string.
Aside: Note that when a namespace declaration (other than for
the default namespace) is represented as an attribute item in
the [namespace attributes], the attribute's [prefix] is
"xmlns", its [local name] is the namespace prefix, and its
[normalized value] is the namespace name.
(2) A canonical namespace prefix is unused if it is not currently the
[prefix] of any namespace item in the [in-scope namespaces] of
the element item. Replace the [local name] of the selected
attribute item with the unused canonical namespace prefix that
has the non-negative number string with the least integer value
(e.g., n2 is less than n10).
(3) The selected attribute item has a corresponding namespace item in
the [in-scope namespaces] of the element. Replace the [prefix]
of this corresponding namespace item with the canonical namespace
prefix determined in step (2).
(4) The element item and its [attributes] property, and descendent
element items and their [attributes] properties, may depend on
the selected attribute item to determine the binding between
their [prefix] and [namespace name]. Replace the [prefix] of any
such dependent element items and attribute items with the
canonical namespace prefix determined in step (2).
Note that a namespace prefix can be redeclared (reused).
Replacement of the prefix does not apply to an element item
wherein the prefix is redeclared, or to the descendants of such
an element item.
(5) The character data translations for values of the QName ASN.1
type may depend on the selected attribute item to determine the
binding between their namespace prefix and namespace name.
Replace the namespace prefix of any such dependent character data
translation with the canonical namespace prefix determined in
step (2).
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Note that a character data translation can appear in the
[normalized value] of an attribute item, or as a sequence of
character items in the [children] of an element item.
6.12. Serialization
The final RXER encoding is produced by serializing the Infoset
translation as an XML document. An implementation MUST serialize the
Infoset translation as an XML document in such a way that the Infoset
of the resulting XML document matches the Infoset translation, after
ignoring the following properties:
(1) all properties of the document item except the
[document element],
(2) the [base URI] of any item,
(3) the [element content whitespace] of character items,
(4) the [notation] of processing instruction items,
(5) the [in-scope namespaces] of element items.
Aside: The [in-scope namespaces] of a parent element item are only
selectively inherited by its child element items in the Infoset
translations of ASN.1 values. This means that the Infoset
reconstructed by parsing the XML document serialization of the
original Infoset will generally have more namespace items in its
[in-scope namespaces], but these extra namespace items will not be
significant.
Aside: A consequence of case (1) is that comments and PIs before
and after the document element are permitted.
In general, there is more than one possible serialization for any
given Infoset translation. Section 6.12.1 highlights some important
considerations in producing a correct serialization and discusses
some of the serialization options.
Section 6.12.2 applies to CRXER encodings and limits the
serialization options so that each distinct Infoset has only one
possible serialization.
6.12.1. Non-Canonical Serialization
This section discusses aspects of Infoset serialization for
non-canonical RXER encodings, but is not an exhaustive list of the
options for non-canonical serialization.
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If one or more character items have a [character code] in the range
U+0001 to U+0008, U+000B to U+000C, or U+000E to U+001F, or one or
more characters in any attribute's [normalized value] are in the
range U+0001 to U+0008, U+000B to U+000C, or U+000E to U+001F, then
the Infoset translation MUST be serialized as an XML version 1.1
document; otherwise, the Infoset translation is serialized as either
an XML version 1.0 or version 1.1 document.
A non-canonical RXER encoding may use any of the allowed character
encoding schemes for XML. RXER encoders and decoders MUST support
the UTF-8 character encoding.
An element item may be serialized as an empty-element tag if it has
no items in its [children].
Attributes of an element can appear in any order since the
[attributes] and [namespace attributes] of an element item are
unordered.
Ampersand ('&', U+0026) and open angle bracket ('
RFC 4910 Robust XML Encoding Rules July 2007
(U+0085), or just a carriage return character (U+000D) provided
the next item is not a character item that is serialized as a
line feed (U+000A) or next line (U+0085) character.
Aside: All these sequences will be normalized to a line feed
character (U+000A) during decoding.
A character item with the [character code] U+000D (carriage return),
U+0085 (next line), or U+2028 (line separator) must be serialized as
a character reference to protect the character from line break
normalization during decoding.
The attribute value normalization performed by XML processors allows
some freedom in how a space character (U+0020) is serialized:
(1) If XML version 1.0 is selected, then a space character (U+0020)
in an attribute item's [normalized value] is serialized as either
a space character (U+0020), a tab character (U+0009), a carriage
return character (U+000D), a line feed character (U+000A), a
carriage return character (U+000D) followed by a line feed
character (U+000A), or just a carriage return character (U+000D)
provided the next character in the [normalized value] is not
serialized as a line feed character (U+000A).
(2) If XML version 1.1 is selected, then a space character (U+0020)
in an attribute item's [normalized value] is serialized as either
a space character (U+0020), a tab character (U+0009), a carriage
return character (U+000D), a line feed character (U+000A), a next
line character (U+0085), a line separator character (U+2028), a
carriage return character (U+000D) followed by a line feed
character (U+000A), a carriage return character (U+000D) followed
by a next line character (U+0085), or just a carriage return
character (U+000D) provided the next character in the
[normalized value] is not serialized as a line feed (U+000A) or
next line (U+0085) character.
Aside: All these sequences will be normalized to a space
character (U+0020) during decoding, through a combination of
line break normalization and attribute value normalization.
Each tab (U+0009), line feed (U+000A), or carriage return (U+000D)
character in an attribute item's [normalized value] must be
serialized as a character reference to protect the character from
attribute value normalization during decoding. In addition, if XML
version 1.1 is selected, then each next line (U+0085) or line
separator (U+2028) character must be serialized as a character
reference.
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Parsed entity references may be used (unless the environment in which
the RXER encoding is used disallows entity references). If entity
references to other than the predefined entities are used, then the
XML document containing the RXER encoding must necessarily contain a
document type declaration, and the internal or external subset of the
document type definition must contain entity declarations for those
entities.
6.12.2. Canonical Serialization
This section discusses Infoset serialization for CRXER encodings.
The serialization of an Infoset for a CRXER encoding is restricted so
that each distinct Infoset has only one possible serialization as an
XML document.
Aside: These restrictions have been chosen so as to be consistent
with Canonical XML [CXML], where possible.
The document SHALL be encoded in UTF-8 without a leading Byte Order
Mark [UCS].
The XMLDecl of the document SHALL be .
A document type declaration (doctypedecl) SHALL NOT be used.
Aside: This has the effect of excluding entity references, except
those for the predefined entities (e.g., &).
A single line feed character (U+000A) MUST be inserted immediately
before the document element.
No other white space characters are permitted before or after the
document element.
There SHALL NOT be any PIs or comments before or after the document
element.
An element item MUST NOT be serialized as an empty-element tag.
Aside: If an element item has no items in its [children], then it
is serialized as a start-tag followed by an end-tag.
There SHALL NOT be any white space characters immediately before the
closing '>' of an element's start-tag and end-tag. The white space
preceding each attribute SHALL be exactly one space character
(U+0020). There SHALL NOT be any white space characters immediately
before or after the equals sign (U+003D) in an attribute.
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The delimiter for attribute values SHALL be the double quote
character (U+0022).
Namespace declaration attributes MUST appear before any other
attributes of an element. A namespace declaration for the default
namespace, if present, MUST appear as the first attribute. The
remaining namespace declaration attributes MUST appear in
lexicographic order based on [local name].
Aside: In particular, this means that xmlns:n10 comes before
xmlns:n2.
The attributes that are not namespace declarations MUST be
lexicographically ordered on [namespace name] as the primary key and
[local name] as the secondary key.
CDATA sections SHALL NOT be used.
Each ampersand character ('&', U+0026) in an attribute item's
[normalized value] MUST be serialized as the entity reference &.
Each open angle bracket character ('
RFC 4910 Robust XML Encoding Rules July 2007
A character item with the [character code] U+000A MUST be serialized
as a single U+000A character.
The white space separating the [target] and [content] in the
serialization of a processing instruction item SHALL be exactly one
space character (U+0020).
Aside: A processing instruction or comment can only appear in a
CRXER encoding if it is embedded in a Markup value.
6.12.3. Unicode Normalization in XML Version 1.1
XML Version 1.1 recommends, but does not absolutely require, that
text be normalized according to Unicode Normalization Form C
[UNICODE]. ASN.1 has no similar requirement on abstract values of
string types, and ASN.1 canonical encoding rules depend on the code
points of characters being preserved.
To accommodate both requirements, applications SHOULD normalize
abstract values of ASN.1 character string types according to Unicode
Normalization Form C at the time the values are created, but MUST NOT
normalize a previously decoded abstract value of an ASN.1 character
string type prior to re-encoding it. An application may, of course,
normalize a decoded abstract value for other purposes, such as
display to a user.
6.13. Syntax-Based Canonicalization
ASN.1 encoding rules are designed to preserve abstract values, but
not to preserve every detail of each transfer syntax that is used.
In the case of RXER, this means that the Infoset representation of an
abstract value is not necessarily preserved when the abstract value
is decoded and re-encoded (regardless of the encoding rules used).
However, syntax-based canonicalization for XML documents (e.g.,
Canonical XML [CXML]) depends on the Infoset of an XML document being
preserved. The Infoset representation of an XML document containing
the RXER encoding of an ASN.1 abstract value potentially changes if
that value is decoded and re-encoded, disrupting the Canonical XML
representation. Extra normalization is required if RXER is to be
usefully deployed in environments where syntax-based canonicalization
is used.
Prior to applying syntax-based canonicalization to an XML document,
any element items in the Infoset representation of the document that
correspond to the value of an ASN.1 top-level NamedType or have
content and attributes that correspond to an ASN.1 abstract value
MUST be replaced by the translation of the value according to CRXER.
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If an application uses Canonical XML but has no knowledge of RXER,
then it will not know to normalize RXER encodings. If RXER is
deployed into an environment containing such applications, then the
Infoset translation for CRXER SHOULD be used for all RXER encodings.
7. Transfer Syntax Identifiers
7.1. RXER Transfer Syntax
The following OBJECT IDENTIFIER has been assigned by xmled.org to
identify the Robust XML Encoding Rules, under an arc assigned to
xmled.org by the Internet Assigned Numbers Authority (IANA):
{ iso(1) identified-organization(3) dod(6)
internet(1) private(4) enterprise(1)
xmled(21472) asnx(1) encoding(1) rxer(0) }
This OBJECT IDENTIFIER would be used, for example, to describe the
transfer syntax for an RXER encoded data-value in an EMBEDDED PDV
value.
7.2. CRXER Transfer Syntax
The following OBJECT IDENTIFIER has been assigned by xmled.org to
identify the Canonical Robust XML Encoding Rules, under an arc
assigned to xmled.org by the IANA:
{ iso(1) identified-organization(3) dod(6)
internet(1) private(4) enterprise(1)
xmled(21472) asnx(1) encoding(1) crxer(1) }
This OBJECT IDENTIFIER would be used, for example, to describe the
transfer syntax for a CRXER encoded data-value in an EMBEDDED PDV
value.
8. Relationship to XER
The Robust XML Encoding Rules (RXER) and the XML Encoding Rules (XER)
[X.693] are separate, distinctly different and incompatible ASN.1
encoding rules for producing XML markup from ASN.1 abstract values.
RXER is therefore unrelated to the XML value notation of X.680
[X.680].
This section describes some of the major differences between RXER and
XER.
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There are essentially two varieties of XER: BASIC-XER (with a
canonical form called CANONICAL-XER) and EXTENDED-XER. The
significant difference between the two varieties is that XER encoding
instructions are used by EXTENDED-XER, but are ignored by BASIC-XER
(and therefore by CANONICAL-XER). There isn't a canonical variant of
EXTENDED-XER. Characteristics that are common to BASIC-XER and
EXTENDED-XER will simply be noted as being characteristics of XER.
Elements and attributes are the fundamental discrete structures of an
XML document. Not surprisingly, schema languages for XML typically
have the means to describe, name, and reference global (i.e.,
top-level) elements and attributes. Global type definitions are seen
more as a convenience for defining the contents of elements and
attributes. Traditional ASN.1 has the means to define global types
(and other global constructs that support the definition of types)
but nothing akin to a global element or attribute definition. The
fundamental difference between RXER and XER is in how this omission
is addressed.
With XER, type definitions are also regarded as being element
definitions by default, or as attribute definitions in the presence
of an XER ATTRIBUTE encoding instruction. In some circumstances an
anonymous Type is required to define an element, which leads to
element names like and . NamedType notation also
defines local elements, and there are some curious cases in
EXTENDED-XER where NamedType notation can define a global type. So
under XER, types can be defined by either Type or NamedType notation,
and elements and attributes can also be defined by either Type or
NamedType notation.
With RXER, types are only defined by Type notation and elements and
attributes are only defined by NamedType notation. Global element
and attribute definitions are made possible by top-level NamedType
notation in an RXER encoding control section.
RXER, with its clean separation of Type notation for types and
NamedType notation for elements and attributes, is a better basis
than XER for translating an ASN.1 specification into an XML
representation (i.e., ASN.X [ASN.X]) or a compatible XML Schema,
where type, element, and attribute definitions are also distinctly
separate constructs.
There is usually a requirement on applications specified in ASN.1 to
maintain backward compatibility with the encodings generated by
previous versions. The encodings in question are typically BER.
Even with the backward-compatibility constraint there is still
considerable leeway for specification writers to rewrite the earlier
specification. For example, they could rename types, factor out an
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in-line type definition as a defined type (or the reverse), or
replace a type definition with an equivalent parameterized reference.
These changes produce no change to BER, DER, CER [X.690], Packed
Encoding Rules (PER) [X.691], or Generic String Encoding Rules (GSER)
[GSER] encodings (so specification writers have felt free to make
such changes to improve their specification), but can change the
names of elements in the XER encoding because XER uses types as
element definitions. The RXER encoding is immune to this problem,
thus RXER encodings are more stable than XER encodings over
successive revisions of an ASN.1 specification (which explains the
first 'R' in RXER). This has an obvious benefit for
interoperability.
RXER has special provisions for encoding values of the QName and
Markup types. QName is used to hold qualified names and Markup can
be used to hold arbitrary untyped markup. XER doesn't recognize any
special types like these, but it is possible to get the same effects
as RXER's QName and Markup types by using XER encoding instructions.
Since CANONICAL-XER ignores encoding instructions, this means that
under XER an application can either support qualified names and
untyped markup, or support canonical XML encodings, but not both. In
contrast, CRXER has canonicalization rules for qualified names and
for Markup. Furthermore, EXTENDED-XER does not address the issues of
normalization of untyped data for other ASN.1 canonical encoding
rules (e.g., for DER; see Section 4.1.2) or normalization of XML
encodings for syntax-based canonicalization (e.g., for Canonical XML;
see Section 6.13).
Both EXTENDED-XER and RXER use encoding instructions to define
attributes, union types, and list types, among other things. Since
CANONICAL-XER ignores encoding instructions, this means that under
XER an application must choose between making use of attributes,
union types, list types, etc., or supporting canonical XML encodings.
In contrast, the canonicalization rules for CRXER encompass all the
encoding instructions for RXER.
9. Security Considerations
RXER does not necessarily enable the exact BER octet encoding of
values of the TeletexString, VideotexString, GraphicString, or
GeneralString types to be reconstructed, so a transformation from DER
to RXER and back to DER may not reproduce the original DER encoding.
This is a result of inadequate normalization of values of these
string types in DER. A character in a TeletexString value (for
example) that corresponds to a specific ISO 10646 character can be
encoded for BER in a variety of ways that are indistinguishable in an
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RFC 4910 Robust XML Encoding Rules July 2007
RXER re-encoding of the TeletexString value. DER does not mandate
one of these possible character encodings in preference to all
others.
Because of the above, RXER MUST NOT be used to re-encode, whether for
storage or transmission, ASN.1 abstract values whose original DER or
CER encoding must be recoverable, and whose type definitions involve
the TeletexString, VideotexString, GraphicString, or GeneralString
type. Such recovery is needed for the verification of digital
signatures. In such cases, protocols ought to use DER or a DER-
reversible encoding. In other cases where ASN.1 canonical encoding
rules are used, values of the Markup type must be normalized as
described in Section 4.1.2.
A transformation from CRXER to BER and back to CRXER does reproduce
the original CRXER encoding, therefore it is safe to use BER, DER, or
CER to re-encode ASN.1 abstract values whose original CRXER encoding
must be recoverable.
Digital signatures may also be calculated on the Canonical XML
representation of an XML document. If RXER encodings appear in such
documents, then applications must normalize the encodings as
described in Section 6.13.
The null character (U+0000) cannot be represented in XML and hence
cannot be transmitted in an RXER encoding. Null characters in
abstract values of ASN.1 string types will be dropped if the values
are RXER encoded; therefore, RXER MUST NOT be used by applications
that attach significance to the null character.
When interpreting security-sensitive fields, and in particular fields
used to grant or deny access, implementations MUST ensure that any
comparisons are done on the underlying abstract value, regardless of
the particular encoding used. Comparisons of Markup values MUST
operate as though the values have been normalized as specified in
Section 4.1.2.
10. Acknowledgements
The technology described in this document is the product of a
research project begun jointly by Adacel Technologies Limited and
Deakin University, and subsequently refined and completed by eB2Bcom.
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11. IANA Considerations
The IANA has registered a new XML namespace in accordance with RFC
3688 [XMLREG].
URI: urn:ietf:params:xml:ns:asnx
Registrant Contact: Steven Legg
XML: None
12. References
12.1. Normative References
[BCP14] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629, November 2003.
[XMLREG] Mealling, M., "The IETF XML Registry", RFC 3688, January
2004.
[URI] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RXEREI] Legg, S., "Encoding Instructions for the Robust XML
Encoding Rules (RXER)", RFC 4911, July 2007.
[ASN.X] Legg, S., "Abstract Syntax Notation X (ASN.X)", RFC 4912,
July 2007.
[X.680] ITU-T Recommendation X.680 (07/02) | ISO/IEC 8824-1,
Information technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation.
[X.680-1] ITU-T Recommendation X.680 (2002) Amendment 1 (10/03) |
ISO/IEC 8824-1:2002/Amd 1:2004, Support for EXTENDED-XER.
[X.681] ITU-T Recommendation X.681 (07/02) | ISO/IEC 8824-2,
Information technology - Abstract Syntax Notation One
(ASN.1): Information object specification.
[X.682] ITU-T Recommendation X.682 (07/02) | ISO/IEC 8824-3,
Information technology - Abstract Syntax Notation One
(ASN.1): Constraint specification.
Legg & Prager Experimental [Page 75]
RFC 4910 Robust XML Encoding Rules July 2007
[X.683] ITU-T Recommendation X.683 (07/02) | ISO/IEC 8824-4,
Information technology - Abstract Syntax Notation One
(ASN.1): Parameterization of ASN.1 specifications.
[X.690] ITU-T Recommendation X.690 (07/02) | ISO/IEC 8825-1,
Information technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER).
[UCS] ISO/IEC 10646-1:2000, Information technology - Universal
Multiple-Octet Coded Character Set (UCS) - Part 1:
Architecture and Basic Multilingual Plane.
[UNICODE] The Unicode Consortium, "The Unicode Standard, Version
4.0", Boston, MA, Addison-Wesley Developers Press, 2003.
ISBN 0-321-18578-1.
[XML10] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E. and
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fourth
Edition)", W3C Recommendation,
http://www.w3.org/TR/2006/REC-xml-20060816, August 2006.
[XML11] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E.,
Yergeau, F., and J. Cowan, "Extensible Markup Language
(XML) 1.1 (Second Edition)", W3C Recommendation,
http://www.w3.org/TR/2006/REC-xml11-20060816, August 2006.
[XMLNS10] Bray, T., Hollander, D., Layman, A., and R. Tobin,
"Namespaces in XML 1.0 (Second Edition)", W3C
Recommendation,
http://www.w3.org/TR/2006/REC-xml-names-20060816, August
2006.
[XMLNS11] Bray, T., Hollander, D., Layman, A. and R. Tobin,
"Namespaces in XML 1.1 (Second Edition)", W3C
Recommendation,
http://www.w3.org/TR/2006/REC-xml-names11-20060816, August
2006.
[INFOSET] Cowan, J. and R. Tobin, "XML Information Set (Second
Edition)", W3C Recommendation,
http://www.w3.org/TR/2004/REC-xml-infoset-20040204,
February 2004.
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RFC 4910 Robust XML Encoding Rules July 2007
[XSD1] Thompson, H., Beech, D., Maloney, M. and N. Mendelsohn,
"XML Schema Part 1: Structures Second Edition", W3C
Recommendation,
http://www.w3.org/TR/2004/REC-xmlschema-1-20041028/,
October 2004.
12.2. Informative References
[GSER] Legg, S., "Generic String Encoding Rules (GSER) for ASN.1
Types", RFC 3641, October 2003.
[X.691] ITU-T Recommendation X.691 (07/02) | ISO/IEC 8825-4:2002,
Information technology - ASN.1 encoding rules:
Specification of Packed Encoding Rules (PER).
[X.693] ITU-T Recommendation X.693 (12/01) | ISO/IEC 8825-4:2002,
Information technology - ASN.1 encoding rules: XML
encoding rules (XER).
[XSD2] Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes
Second Edition", W3C Recommendation,
http://www.w3.org/TR/2004/REC-xmlschema-2-20041028/,
October 2004.
[CXML] Boyer, J., "Canonical XML Version 1.0", W3C
Recommendation,
http://www.w3.org/TR/2001/REC-xml-c14n-20010315, March
2001.
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RFC 4910 Robust XML Encoding Rules July 2007
Appendix A. Additional Basic Definitions Module
This appendix is normative.
AdditionalBasicDefinitions
{ iso(1) identified-organization(3) dod(6)
internet(1) private(4) enterprise(1)
xmled(21472) asnx(1) module(0) basic(0) }
-- Copyright (C) The IETF Trust (2007). This version of
-- this ASN.1 module is part of RFC 4910; see the RFC itself
-- for full legal notices.
--
-- Regarding this ASN.1 module or any portion of it, the authors
-- make no guarantees and are not responsible for any damage
-- resulting from its use. The authors grant irrevocable permission
-- to anyone to use, modify, and distribute it in any way that does
-- not diminish the rights of anyone else to use, modify, and
-- distribute it, provided that redistributed derivative works do
-- not contain misleading author or version information.
-- Derivative works need not be licensed under similar terms.
DEFINITIONS
RXER INSTRUCTIONS
AUTOMATIC TAGS
EXTENSIBILITY IMPLIED ::= BEGIN
Markup ::= CHOICE {
text SEQUENCE {
prolog UTF8String (SIZE(1..MAX)) OPTIONAL,
prefix NCName OPTIONAL,
attributes UTF8String (SIZE(1..MAX)) OPTIONAL,
content UTF8String (SIZE(1..MAX)) OPTIONAL
}
}
AnyURI ::= UTF8String (CONSTRAINED BY
{ -- conforms to the format of a URI -- })
NCName ::= UTF8String (CONSTRAINED BY
{ -- conforms to the NCName production of
-- Namespaces in XML 1.0 -- })
Name ::= UTF8String (CONSTRAINED BY
{ -- conforms to the Name production of XML -- })
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RFC 4910 Robust XML Encoding Rules July 2007
QName ::= SEQUENCE {
namespace-name AnyURI OPTIONAL,
local-name NCName
}
ENCODING-CONTROL RXER
TARGET-NAMESPACE "urn:ietf:params:xml:ns:asnx" PREFIX "asnx"
COMPONENT context [ATTRIBUTE] [LIST] SEQUENCE OF prefix NCName
END
Authors' Addresses
Dr. Steven Legg
eB2Bcom
Suite 3, Woodhouse Corporate Centre
935 Station Street
Box Hill North, Victoria 3129
AUSTRALIA
Phone: +61 3 9896 7830
Fax: +61 3 9896 7801
EMail: steven.legg@eb2bcom.com
Dr. Daniel Prager
EMail: dap@austhink.com
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RFC 4910 Robust XML Encoding Rules July 2007
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